I imagine you would have to use something like (thanks to 
dt):
code:
var theRECT = Interop.Allocate(16); //long = 4 so size = 4 * 4//
Interop.Call("user32", "GetWindowRect", PlusWnd.Handle, theRECT);
    var left = theRECT.ReadDWORD(0); //left
    var top = theRECT.ReadDWORD(4); //Top
    var right = theRECT.ReadDWORD(8 ); //Right
    var bottom = theRECT.ReadDWORD(12); //Bottom
var WndWidth = right - left;