how child form is completely fitted between menustrip and statusstrip in MDI form

I have certain problem regarding the position of child form when it is call from a MDI form.

I have written a code when i click on menu.

Dim x as New frm1

x.StartPosition = formstartPosition.centerParent

x.showdialog

But when i write this code the form will open in center position, but i want that it should be shown in whole MDI within MenuStrip ans StatusStrip.

If any body knows the answer, plz reply

Thanks in advance