Stems range in size fairly dramatically, from 0mm on up. They are important because, otherwise, your bike would only be connected to the fork via the cables. Ha ha. No, stem length needs to be adjusted according to your riding style and physical build. You'll want to consult a LBS (local bike shop) since you, apparently, don't have much experience in this area - there are some great ones in Portland. One other thing; stems also come in varying degrees of "rise" which allows you to further tweak handling and ergonomics.