Dumb question. I haven't ridden a mountain bike since I was about 15, but I'm scouting CL for one to convert to a SS for winter bad weather commuting. How do you know what size get? I assume that MTBs with a flat top tube are sized more or less the same as road bikes, but what about ones with sloping top tubes? I also plan to put mustache bars on it. Should I get a longer top tube than normal to accommodate the backward sweep?