[Jacob Bendicksen]

I'm not familiar with SAI, but as for Photoshop, you'd probably be better off using Adobe Illustrator, InDesign, or something similar. While Photoshop uses pixels, both Illustrator and InDesign use vectors, meaning that what you make is infinitely scalable - we use the exact same file for our pit banner and buttons. To answer your questions, save them in a vector format (.ai for Illustrator and .indd for InDesign), and you don't need to worry about how big to save them - with vectors, it doesn't matter.

PM me if you want to see what our graphics files look like - I'd be happy to send them your way.