Hello. I've had a quick look around the forum and cannot find an answer to this question...I apologise if there is one and i've not found it. How do you actually size the bike up that you require...I'm 5'6" and have an inside leg of 31". I know there is a relation to the distance "bum to pedal and bum to floor"....but how and what do you do to get the correct one for you. Sorry if this seems a very basic and stupid question.