Most Toys came with tires such as 235/75/15. Newer trucks may have gone to a 16 inch rim, but the overall dimensions are basically the same. These tires are fine for the casual offroader, but most of us will want something bigger.
Since tires are the only true way to get ground clearance, most people will say the taller the better. That's fine, but keep in mind that the Toyota was not made to run 44 inch tires with out modification. Many people are able to fit 35's under thier Toys with body lifts and a mild suspenion lift. However, it's generaly accepted that anything bigger than 33's cause increased stress on the components. Particularly if you have a soild front axle with the origianl Birfield Joints. You can beef up your axle, but that takes money.
I run 33x12.5 - 15's on my toy and have never had a problem. Our V8 truck runs 35's and also has never had a problem. If pushed, who knows? I'll put my money that the 35's break something before the 33's.
There is also the issue of width. The taller you go, the higher the center of gravity. Our 35" equipped toy is much less stable than the 33". Do you really want 40" tires with the track of a toyota truck? Axle swaps can give you the extra width to keep your big tired rig stable. $$$$$$$$
When ordering an aftermarket rim for your truck you can get some extra track width by getting rims with lots of offset. ie. Put the tire away from the axle as much as possible. My blue truck has loads of offset in it's wheels. It's a cheap way to get the width, but it only goes so far.
And tread design? The more agressive it is, the more you'll do.