I wouldn't say that 33s are large, but to answer your question I think its either 35s or 37s that you can fit onto the tailgate without too many issues, but just consider that weight hanging on your tailgate and all the bumps and thumps you are driving over. I would recommend either a rear bar with a tyre carrier or at least a Teraflex HD tyre carrier that also upgrades the hinges on the tailgate too.
The old adage of spending more to get better isn't always true. So just research thoroughly before you buy. Talk to people who have those tyre on and ask them where they drive - in the mud and sand or just to the local shops.
Overall your decision on size may also be affected by how large you want to go and what "extras" you are prepared to add to the vehicle to "cope" with the new rubber - diff ratios etc. Im one of the lucky Rubicon drivers who can throw on 33s or 35s without having to 'really' worry about ratios.
its a can of worms....
