I'm working on submitting an online art portfolio for MICA's scholarships, and I was wondering if there are any general size limitations. If any one knows the how that applies specifically for MICA's online portfolios, that'd be nice, too.

Say, would something like 480 x 640 pixels be too small?

I would suggest contacting the school/program directly as they would know best. Better to ask than rely on a possibly incorrect answer from the collective. I personally find that size too small, but am not an expert in applying to this program.