For normal 96 DPI res, we use MS Sans Serif font. When a user changes to large font, we have all controls changing automatically to a scalable font (code is in our base classes). So, for larger fonts, we use Tahoma because it's in the mood, but Arial works too.

Then, you will have to test all your forms on a PC with large fonts and adjust positions to make sure controls don't overlap.

Tip: test you app on a PC with large fonts, but DON'T modify your form on the same PC, if possible. Go back to small fonts befor you adjust the controls positions. If you modify your form on a PC with large fonts, your form will look weird when you go back to small fonts...