Set design size for WPF ControlTemplate in Blend

I've been looking for a way to set the "design size" for ControlTemplates in Blend, similar to the way you can set DesignWidth and DesignHeight for Windows and Pages. I have a custom control I'd like my UI designer to be able to edit in Blend, but the design space is the size of a button's, and I want it to be a different aspect ratio. I've noticed that if I change the TargetType of the template the design size will change - a Control is 20x20, a Button is 125x35, etc. Is there some value I can set that will give my custom control a default size of some sort?

Let me know if I need to explain further. I'm creating a WPF application using C#, and I'm using VS 2012 + Blend For Visual Studio.