First off, get a rocket that tickles your fancy. If you don't like the look of it, you won't want to fly it.
Second, the size of your launch field will drive your kit choice. Do you have nothing but flat sod in all directions? You could go ahead and get a 54mm rocket and plan to walk a ways for recovery. Do you have a small field ringed with swamps filled with rocket-eating trees and rocketeer-eating alligators? You definitely want a 4" rocket so it doesn't go as high. Personal preference will play a role here as well.
Third, I'd suggest a 2.6" or 3" diameter heavy wall cardboard rocket with a 29mm motor mount will be a really versatile flyer. It should go well on a G or even possibly a sporty F, but will still be fairly easy to see the full flight on an H. You probably won't fly it on more than a smallish H, but you'll learn a lot for when you want to build something bigger, faster, louder, and higher. My L1 cert was a 54mm rocket with a 29mm motor mount. I think I still fly it more than any of my other rockets.
LOC, Madcow, and others all have great options in this size range in a variety of styles. And everyone else in the thread has given good advice.