Depends on the lug arrangement whether you can go 16" or not. The only come in 6 lug or higher IIRC.

mostly depends on your clearance on your fenders as to what you can get away with. go as big as you can

In case of running with cooler bearings, this is true. In terms of load, not so. Almost all 235/80-16 E capacity trailer and truck tires has a max load between 3400-3500 lbs. The 235/85-16 E capacity tires can "only" load 3000-3100 lbs. For the Goodyear Marathon and Loadstar + most truck tires this is the case. The only tires I know of that differs, are the Kenda Karrier HD F rating and Gladiator Special in 235/85R16 rated to 3,960 lbs.