There are many awesome tips, choices, and professionals on this topic, but for the sake of my thoughts, I'm keeping it "simple". My goal is to pass on a tip that has worked for ME, and hopefully will work for you.
One of the first things is to determine if you are going to BOWHUNT, spot target shoot, 3D shoot, or a combination of.
I'm going to focus on the bowhunting side of things. In my opinion, the same size should be used for hunting and 3D shooting. Why do I say this? By choice, I like to keep my real hunting shots to less than 30 yards (and that's pushing it). 3D shooting, it's ok to take long shots at the animal targets (they will live, baha). The key is to be able to have NO distortion, most view of your target when at full draw/aiming, and the ability to shoot under LOW-LIGHT conditions. Too, my larger diameter peep (1/4") allows me to see more of the contrast of my target versus the background (especially in wooded, thicker areas) and to read my target's body language. If your peep creates a "ring in a ring" effect (when the peep is aligned perfectly around your sight housing at full draw), your heading in the right direction. My peep aligns well with my HHA Optimizer.
With today's technologically advanced bows, we still need to keep ourselves in tip-top tuned form - not just physically (muscle memory, hand torquing, back tension, etc.), but also with our accessories. After all, the "bow is an extension of the archer", it's part of us. And like I always say (Thank you coach...), "practice the way your gonna play!"
Remember: right equipment equals confidence = accuracy = success = FUN! (good one, hey?)
Like with everything else, test out a variety of styles & sizes. Hunting? You don't want an obscured target, we owe it to our game the best shot possible. GOOD LUCK picking the right peep and size. Happy Shooting!
(If you have other tips, please do share, I love to learn other tips).