4" wide 10 mm thick for squats
Tapered belt 4" in back, 2" in front for conventional deadlifts.
In the bottom of the conventional deadlift many people find that the belt gets pushed out of position by contact with the thighs. If you deadlift sumo, then there is no need for the tapered belt.
If you have extra cash a 2" all around belt for a shirted bench (if you use a shit) to hold the shirt in place and narrow so it doesn't interfere with the arch.
Either way buy a suede belt (wont slip), with one or two prongs (preference) and don't be cheap. Velcro and single ply bodybuilding belts are a waste of money. A good belt will last your entire career unless you grow out of it. any of the major vendors (Inzer, Titan, Metal, Rogue, etc) are fine with Bob Morris generally considered the cream of the crop.