No, a 20 gal. is definitely not "good for life"- BPs average about 4' as adults- & some (esp. females) get quite a bit bigger. If you stuck with glass tanks, a 40-gal. breeder tank would be more the adult BP size. Most here prefer plastic enclosures, but you can make it work either way.
Best to do research before going into pet stores- they're often incorrect- not always, but remember they're trying to make a sale, so if nothing else, they usually make everything sound easier than it really is. Be skeptical & do your own research. Some pet store employees are wonderful, many just mean well but don't have their own experience, & most just say what they're supposed to say regardless.Even among very experienced snake-keepers (including here)- understand that we all don't agree anyway- at some point, the choices are yours to make & live with. But we'll be happy to help by discussing options, including the pros & cons.