31 is the biggest I would try.
Looks good until you get stuck and can't get the tailgate down, Or the rear window motor gives outhere are some pics. of the platform I made to cover my tire and also my box with some trail essentials (strap, chain etc.) I have an axe and shovel that fit beside the tire. this setup is temporary untill I build my drawer, then the platform will be attached to the floor and covered with speaker box carpet.
here is the link to the pics.