I have heard that you can go as big as a 21 x 1.5 without much discomfort. These are usually the smallest one can find at the local farm supply. They work well drawing the thick stuff too if you only have one size available. While you're there, spend an extra 30 cents and get an 18 ga to draw with if you're feelin' like a big spender. As a newby, you may want to start with the quads. The glutes aren't tough to hit, but I have heard it can be tough to hold everything real still during the shot which can result in a real sore butt. The better you are at keeping still, the less sore you'll be. Otherwise, I've heard all the advice you found on the web site you mentioned is really good. Ready, aim...
"Don't shoot him. You'll just piss him off."