How to enlarge a textbox control when it is mouse-overed?

What is the question? Are you asking how to handle the mouse-over event or are you asking how to enlarge the textbox? Those are two different questions.

You could enlarge the textbox by changing its size and position. I don't know how to determine how large the text is, in pixels, but if you can figure ou thow to determine that then you can use either the SizeFromClientSize Method or the SetClientSizeCore Method; I don't know what the difference is between them. The description of the SizeFromClientSize method is very vague.