Keyboard size is absolutely a defining factor. Full-sized keyboards tend to offer the most features and a Numpad, but if you don't have space, then all of those extras you paid for will be useless. Tenkeyless boards (the ones with no number pad) and compact keyboards can be a great option, too, if you don't care about all the extra bells and whistles or you don't have any use for alt codes (how barbaric!).