ON SUBJECT: Your LBS owner is a moron, once again.
You can run a 30 tooth sprocket if you want (actually, that would probably hit the brakes if they are chain stay mounted)... Anyways, you can run anywhere from about 42 to 46 without a drastic change in how you pedal. The more teeth, the harder it is to pedal, but the faster you go. The less teeth, the easier it is to pedal, but the slower you go.
Add more teeth, and you may need another link in your chain.
Take teeth away, and you may need to take a link out of your chain.
Over ONE tooth though, you probably can just switch it and it won't really affect your chain that much.