You'll need to be a little more specific. What's the resolution of the vertical games that look small?
You also need to beaware, that no matter what setup you use, be it ArcadeVGA or AdvanceMAME, vertical games on a horizontal monitor will always look like crap. The only exception would be kortek or wellsgardner multisync arcade monitors using AdvanceMAME. Otherwise, vertical games will typically look bad -- stretching, interlacing, dropped lines, out of vsync. There is no perfect solution to vertical games on a standard resolution arcade monitor, only comprimises.