The form is not measured in
millimeters as A4! The question makes no sense. If you want to rely upon some fixed size it won't work properly anyway. I think you need it for printing. You need to design your print document the way if fills in the page property and closely resemble the form.
Look some recommendation on resolution-tolerant design:
Zom Out malfunctions when Screen resolution changes
GUI Apperance - C#.Net
Code behind class implementation
You need to know how to resize a Form of course :-) :