The size number is largely useless (56 vs 58). You need to look at the bikes geometry: effective top tube, head tube length, and seat angle. Know the numbers on the bike that works, and then compare other models to that. Some companies publish stack and reach, which then make models incredibly easy to compare. I would say that if your 58 Trek is comfy, then look at its numbers, and use them as a baseline from there. Helps to know your stem length as well, because then you know how much you can vary that as well.