Hey there W84Dirt! Welcome! (though I'm hardly veteran enough for that hold much weight). Congrats on the new LT!
I'll try my best to answer the questions you posted, but it might make more sense to post a new introduction thread, and a couple of photos of your rig - with some of your questions, as I'm not sure many people will read this deep on a separate discussion.
A) 20in wheels aren't anathema to wheeling - but they are certainly less common. A few guys on the forums really like them, and use them off road - however I would suggest the majority prefer 17in. The wheels themselves are quite a bit lighter than 20's (all things considered equal) - which is important not only for unsprung weight for fast wheeling, but for decreasing the moment of inertia when hammering on / off on difficult or intermittently grabby terrain. The increased rotating mass of a 20in puts more strain on the driveline (when it stops quickly / grabs terrain in a jolt etc.), and of course that leads to more breaks / wear (cv axles / diffs). If you can find a 20in wheel / tire combo that is lighter than a 17in wheel / tire combo with the same grip etc. for your intended purpose - then the above is of course not true - however, many would suggest that finding such a combo would be quite difficult.
B) 17in wheels are for the above, and maybe a few other considerations much more common, as are the availability of tires at local tire shops. 16in wheels are ok as well, but limit your choices in braking. If you are crawling, then you might not need as much brake bite (and if that is your gig, then wilwood makes an incredible lightweight setup that looks like it is off a mountain bike), but for everything else - (again, all things equal) - a larger diameter rotor performs better. Better brake cooling which leads to less fade, more surface area which leads to larger pads, which leads to lower pressure on the pad / hydraulic lines etc. for the same amount of stopping power. Higher pressure means higher heat, and heat is the enemy of brake performance unless you are into some truly exotic materials.
17in is a nice balance of the considerations in A and B, but obviously - there are reasons to go either way. some guys go full retard and get tiny wheels with huge tires and sidewalls - others go the opposite way for huge brakes (see the new fully carbon fibre wheels on the mustang gt350.... yum yum).
As to your last point - there are a few great places on the net for reading, but there really is no substitute for experience. Best bet would be to find a few people locally (which is pretty easy - very friendly crew coast to coast) - and go wheeling. The locals will know the spots for newbies
