Drumsticks are sized according to the diameter of the stick and the shape of the tip. The smaller the number, the larger the diameter. An iAi tip is more slender and pointed than a iBi tip, which is rounder. Although sticks are a personal item, if you’re like most people, a size between 2B and 5A is a good starting point. Your best bet is to go into a store and try a bunch to see which size you prefer. If you’re a rock drummer, try to resist the temptation to get the heaviest sticks. You’ll have a hard time playing anything other than loud, straight-ahead rock music.
As for wood or nylon tipped sticks, this is really personal preference. Nylon tips produce a crisper, brighter sound on the cymbal while the wooden tip variety has a mellower sound.