I'm not sure exactly which emu you're talking about. They are all able to change screen size (Snes, NES, GBX). SNES loses major framerate if you mess with the screen size. NES works beautifully even at full screen. I believe GBx emu does the same, I haven't played around with that one too much.
How do I make the screen bigger? I just got the little one in the center. Im using the Nester J emu for NES. Also, is it possible to change the button configurations, its kinda hard to use the config the way it is. And if there is a better emu out there that I should get instead, feel free to suggest them.