The first red tail I bought, the pet store tried convincing me that it would grow up to 30 feet, and eat on large rabbits several times a week. Granted, the examples aren't always to this extreme, but it's a good idea to KNOW if what they're saying is fact or crap. Generally, it's a good idea to nod your head and smile at these 'know alls.'
Jeff is definately right, before you bring your new herp home, check it out on the internet. Study on the species itself, find a few care sheets and if you can, ask questions on message boards. There's plenty of helpful and knowledgeful people out there. Leave the pet store junkies to their dreams of 30'' red tails...