I’m going to get an old rigid-frame mountain bike for use mostly as a city commuter, and also for gravel road excursions and the occasional road tour. What size should I look for? I’m 6 feet tall but have a relatively short 30” inseam. I consider 58cm/23” my normal road bike size. The MTB sizing recommendations seem very small and they must be calculated with off-road riding in mind: according to one chart I should get a 19” frame. I suppose they have slightly higher BB’s to begin with, but I figure the smaller wheels make up for that. Shouldn’t I look for something larger for road riding? Thanks for your advice.