I would be very surprised to see any cory more than 3-4", and that is just a few varieties. Your average peppered, for instance, will max out at about 2-2 1/2." Pygmys do really stay small. BN are very cool looking, especially the males with their wild tentacles on their noses, and 6" would be a really hefty one, most are around 4 1/2." I have 3 in my 55, but only one male.