WPF UserControl Design Time Size

For Blend, a little known trick is to add these attributes to your usercontrol or window:

 xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
mc:Ignorable="d"
       d:DesignHeight="500" d:DesignWidth="600"

This will set the design height and width to 500 and 600 respectively. However this will only work for the blend designer. Not the Visual Studio Designer.

As far as the Visual Studio Designer your technique is all that works. Which is why I don't use the Visual Studio Designer. ;)

In Visual Studio add the Width and Height attribute to your UserControl XAML, but in the code-behind insert this

public UserControl1()
{
    InitializeComponent();
    if (LicenseManager.UsageMode != LicenseUsageMode.Designtime)
    {
        this.Width = double.NaN; ;
        this.Height = double.NaN; ;
    }
}

This checks to see if the control is running in Design-mode. If not (i.e. runtime) it will set the Width and Height to NaN (Not a number) which is the value you set it to if you remove the Width and Height attributes in XAML.

So at design-time you will have the preset width and height (including if you put the user control in a form) and at runtime it will dock depending on its parent container.

Hope that helps.