I don't suggest the Pure Plugs straight off. They're metal, and, while very comfortable, they are very unforgiving.
is well liked around here, though I don't own it. Joe Rock
is also a good choice.
Basically, you're looking for about the 1.25" in diameter range. You could go up to 1.5" in diameter, but no larger than that, as a beginner. Warm up a lot if you go above 1.25" in diameter.
Stick with silicone--it's soft, forgiving, and generally comfortable.