If you had problems with animating gridcolumn widths or gridrow heights i have a solution for your problems. put the following class somewhere in your project:

using System.Windows;
using System.Windows.Controls;
using System.Windows.Media.Animation;

namespace AltinetSilver.Animations
{
    public static class GridLengthAnimation
    {

        //you only have to set the rowdefinition or columnDefinition!
        #region Element (Attached DependencyProperty)

        public static readonly DependencyProperty ElementProperty =
            DependencyProperty.RegisterAttached("Element", typeof(DependencyObject), typeof(DoubleAnimation), new PropertyMetadata(new PropertyChangedCallback(OnElementPropertyChanged)));

        public static void SetElement(DependencyObject o, DependencyObject value)
        {
            o.SetValue(ElementProperty, value);
        }

        public static DependencyObject GetElement(DependencyObject o)
        {
            return (DependencyObject)o.GetValue(ElementProperty);
        }

        private static void OnElementPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
        {
            if (e.NewValue != null)
            {
                DoubleAnimation timeAnimation = (DoubleAnimation)d;
                var target = e.NewValue;
                timeAnimation.SetValue(TargetProperty, target);
                DependencyProperty targetProperty;
                GridUnitType gType;
                if (target is ColumnDefinition)
                {
                    targetProperty = ColumnDefinition.WidthProperty;
                    gType = ((ColumnDefinition)target).Width.GridUnitType;
                }
                else
                {
                    targetProperty = RowDefinition.HeightProperty;
                    gType = ((RowDefinition)target).Height.GridUnitType;
                }
                timeAnimation.SetValue(TargetPropertyProperty, targetProperty);
                timeAnimation.SetValue(TargetUnitTypeProperty, gType);
                Storyboard.SetTargetProperty(timeAnimation, new PropertyPath("(GridLengthAnimation.Time)"));
                Storyboard.SetTarget(timeAnimation, timeAnimation);
            }
        }

        #endregion


        // The time along the animation from 0-1
        public static DependencyProperty TimeProperty = DependencyProperty.RegisterAttached("Time", typeof(double), typeof(DoubleAnimation), new PropertyMetadata(OnTimeChanged));
        // The object being animated
        public static DependencyProperty TargetProperty = DependencyProperty.RegisterAttached("Target", typeof(DependencyObject), typeof(GridLengthAnimation), null);
        public static DependencyProperty TargetPropertyProperty = DependencyProperty.RegisterAttached("TargetProperty", typeof(DependencyProperty), typeof(DependencyObject), null);

        public static DependencyProperty TargetUnitTypeProperty = DependencyProperty.RegisterAttached("TargetUnitType",
                                                                                                      typeof(
                                                                                                          GridUnitType),
                                                                                                      typeof(
                                                                                                          DependencyObject
                                                                                                          ), null);

        /// <summary>
        /// Silverlight's animation system is animating time from 0 to 1.  When time changes we update the grid length to be time
        /// percent between from and to
        /// </summary>
        private static void OnTimeChanged(DependencyObject sender, DependencyPropertyChangedEventArgs e)
        {
            DoubleAnimation animation = (DoubleAnimation)sender;
            double time = GetTime(animation);
            DependencyProperty targetProperty = (DependencyProperty)sender.GetValue(TargetPropertyProperty);
            DependencyObject target = (DependencyObject)sender.GetValue(TargetProperty);
            GridUnitType gType = (GridUnitType) sender.GetValue(TargetUnitTypeProperty);
            target.SetValue(targetProperty, new GridLength(time, gType));

        }

        public static double GetTime(DoubleAnimation animation)
        {
            return (double)animation.GetValue(TimeProperty);
        }

        public static void SetTime(DoubleAnimation animation, double value)
        {
            animation.SetValue(TimeProperty, value);
        }
    }
}

And the usage model would look something like this

            <VisualStateGroup x:Name="NotesStateGroup">
                <VisualState x:Name="NotesVisible">
                    <Storyboard>
                        <DoubleAnimation From="0" To="4" Duration="0:0:0.15" Animations:GridLengthAnimation.Element="{Binding ElementName=Column2}"/>
                    </Storyboard>
                </VisualState>
                <VisualState x:Name="NotesCollapsed">
                    <Storyboard>
                        <DoubleAnimation From="4" To="0" Duration="0:0:0.15" Animations:GridLengthAnimation.Element="{Binding ElementName=Column2}"/>
                    </Storyboard>
                </VisualState>
            </VisualStateGroup>

The important part is in DoubleAnimation tag where you set the extended property GridLengthAnimation.Element where column2 is a name of a column. 3 and 0 are starred values. but with some tweaking of this simple model, you can basically do whatever you want.

UPDATE: I have updated the code, now it supports both starred and fixed column/row widths/heights.
Enjoy!

UPDATE 2 I’ve put a more complete code for one of the controls here as per request in the comment.

<UserControl x:Class="AltiAgent.PartnerPage.PartnersView"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d"
    d:DesignHeight="700" d:DesignWidth="1300" xmlns:Animations="clr-namespace:AltiSilverlight.Animations;assembly=AltiSilverlight" x:Name="PartnersUserControl"
             Animations:StateManager.CurrentState="{Binding CurrentVisualState}"
             >
    <Grid HorizontalAlignment="Stretch" VerticalAlignment="Stretch">
        <VisualStateManager.VisualStateGroups>
            <VisualStateGroup x:Name="NotesStateGroup">
                <VisualState x:Name="NotesVisible">
                    <Storyboard>
                        <DoubleAnimation From="0" To="400" Duration="0:0:0.15" Animations:GridLengthAnimation.Element="{Binding ElementName=Column2}"/>
                    </Storyboard>
                </VisualState>
                <VisualState x:Name="NotesCollapsed">
                    <Storyboard>
                        <DoubleAnimation From="400" To="0" Duration="0:0:0.15" Animations:GridLengthAnimation.Element="{Binding ElementName=Column2}"/>
                    </Storyboard>
                </VisualState>
            </VisualStateGroup>
            <VisualStateGroup x:Name="DetailsStateGroup">
                <VisualState x:Name="DetailsVisible">
                    <Storyboard>
                        <DoubleAnimation From="0" To="300" Duration="0:0:0.15" Animations:GridLengthAnimation.Element="{Binding ElementName=Column1}"/>
                    </Storyboard>
                </VisualState>
                <VisualState x:Name="DetailsCollapsed">
                    <Storyboard>
                        <DoubleAnimation From="300" To="0" Duration="0:0:0.15" Animations:GridLengthAnimation.Element="{Binding ElementName=Column1}"/>
                    </Storyboard>
                </VisualState>
            </VisualStateGroup>
        </VisualStateManager.VisualStateGroups>
        <Grid.ColumnDefinitions>
            <ColumnDefinition x:Name="Column0" Width="*" />
            <ColumnDefinition x:Name="Column1" Width="300" />
            <ColumnDefinition x:Name="Column2" Width="400" />
        </Grid.ColumnDefinitions>
        <ContentControl x:Name="PartnerGridPart"/>
        <ContentControl Grid.Column="1" x:Name="PartnerForm"/>
        <ContentControl Grid.Column="2" x:Name="TaskNoteListPart"/>
    </Grid>
</UserControl>