D7: How to determine size of client are in MDI application?

Hello,

I create a new MDI Form (Form1) and a ChildForm (Form2). After that I
put a button on Form1 which has the following implementation:

form2.Left :=0;
form2.Top := 0;
form2.Width := self.ClientWidth; // self == form1
form2.Height := self.ClientHeight;

When I press the button form2 is resized but after resizing it is a
little bit to large for the client area of the parentform. So how do I
determine the correct size of the client area?

Thank you,
Markus