jodi,
Depending on the breed of cory you're looking at, about 2-3 inches. I've only had Julii cories but I quickly scanned different breeds on planetcatfish and didn't see one listed over 3 inches. I seem to remember there is a cory that got a little over 3 inches but I could be wrong. And they do like to be in groups. I started with two, but I've read that six is a better number. HTH