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?

Take a look at this sizing chart, for a mtb that you're going to ride on the road, you'll want one that's somewhere inbetween your ideal road and mountain sizes:


Compared to road bikes, mountain bikes have much shorter seat tubes (so that the back end of the bike can bounce underneath you as you go over rough ground with your weight on the pedals), and put the rider further behind the center of the cranks. If you buy a MTB with the same size seatpost as you'd want on a road bike, you'd end up with an extremely long reach to the bars, and if you buy a mountain bike that's the right size for trail riding you'd need an extremely long seatpost for efficient use on the road (and you'd end up with an extremely short top tube). So go for something that's between your ideal road/mtb sizes, you'll probably need a long zero-setback post and a tall stem too.

This thread in C&V is full of converted mountain bikes (my '84 Stumpjumper is in there) to look at for inspiration and guidance: Show Your Vintage MTB Drop Bar Conversions

Practically all mountain bikes have sloping top tubes these days. The method for sizing them is similar to the method for sizing road bikes: get the right top tube length and head tube height for your riding position. Nominal seat tube length is less and less important now that bikes are coming with lots of different top tube and head tube lengths.

smaller if doing Gnarly single track , larger if touring and commuting on the pavement.

The size in MTB is different in ROAD bike.if you want to buy the MTB bike ,you can consult with the sell.