I would use custom code.
In a partial class associated with your shape, override the properties MaximumSize and MinimumSize in order to fix their value to the size that you set.
public override Microsoft.VisualStudio.Modeling.Diagrams.SizeD MinimumSize
{
get { return this.Size; }
}
public override Microsoft.VisualStudio.Modeling.Diagrams.SizeD MaximumSize
{
get { return this.Size; }
}
Rgds,
Pascal
http://www.netfxfactory.org/