Remember...........on a road bike, standover clearance doesn't matter as much as the top tube length for comfort on long rides. On an MTB however, standover clearance takes precidence during short stops. Translation..........unexpected sudden dismounts require room.

Most modern MTB's are disigned with a sloping top tube (many with a great deal of slope). This provides some extra clearance. In general, go with the same length virtual top tube as your road bike fit and you will be fine. If the bike is too big, you will have difficulty shifting your weight around on the saddle (and behind it) for off road control.