The change the width when the maptip is expanded, you have to style the maptip control.In Blend, right click the control, create a copy of the Template.In the maptip style, change the width value in the storyboard. Something like:<VisualState x:Name="Expanded">
<Storyboard>
<DoubleAnimationUsingKeyFrames BeginTime="00:00:00" Storyboard.TargetName="MapTipData" Storyboard.TargetProperty="Height">
<SplineDoubleKeyFrame KeyTime="00:00:00.5" Value="150" KeySpline="0.3,0 0,1" />
</DoubleAnimationUsingKeyFrames>
<DoubleAnimationUsingKeyFrames BeginTime="00:00:00" Storyboard.TargetName="MapTipData" Storyboard.TargetProperty="Width">
<SplineDoubleKeyFrame KeyTime="00:00:00.5" Value="300" KeySpline="0.3,0 0,1" />
</DoubleAnimationUsingKeyFrames>
</Storyboard>
</VisualState>