In an MDI application, the MDI child's MaximumSize and MinimumSize properties don't seem to take effect. How can I restrict the size of my MDI child?
It appears that this behavior is a bug that will be corrected in a future .NET release.
You can control the size of your child form by adding a Layout event handler for it. Here is a code snippet that imposes the minimum size that you set in its properties. You can also handle it by overriding the form’s WndProc method as explained in this Microsoft KB article