I've been wanting to get a Trek/GF Mamba for a while now and I found one locally for $700 OBO. When I test road a Marlin, before deciding it would be better to go with a Mamba, I rode a 15.5" and when I stand over it I'm basically touching the top tube. I know 29ers are taller out of necessity but I didn't know if the bike would just be too big then. My road bike is a Trek 1000 and it's a 52 cm. I know when I checked geometry between by 52cm and a 15.5" they were about comparable. So would it be a bad idea to get a bike that's one size bigger?

Thanks in advance!