How do I know the size of the form after the size changed?

Look into the '.SizeChanged' event for the form.
In the function that you define for this event, look into the '.Width' and '.Height' property for the form.

Can I add an icon in the menu (for Exit)?

I have not done that myself and I don't think Shawn has built that in as of yet. If you can figure out how that would be great and others can learn from your example.
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KiXforms Designer .NET 2.0 (Beta)