If the stud is too long, it may get caught on your teeth, or at least tap against them, along with your gums. I can't stand it when that happens -- drives me crazy. As for the first part of your question, when I had rings in my lip, I never bit them unless it was intentional. This, however, was common, as I develop nervous habits efficiently. 
I'm not sure if its really a problem, but I noticed that my lips would get chapped pretty often with rings in. You're looking for a stud, so I don't think it'd be an issue for you. It was almost certainly the habit I mentioned earlier that caused the dryness, but I figured I'd toss it out anyway.