I asked my kids this morning. They said they had only ever seen "twerp" in books.

They had never heard of "twit."
They did know what pipsqueak and runt meant, but didn't think they were commonly used.
They said that bullying is not prevalent in their schools (not that it's not there, because one has even had personal experience, but it's just not prevalent) so they suggested you talk to someone at a school where this is a problem.

(They have some friends who have transferred from schools that they consider much worse.)
They did say that in general kids are more likely to be teased about some feature other than height. Most common insult would be calling someone a "cry baby." And usually kids who were insulted for another "feature" also cried a lot.
American Girl has excellent books on friendship and bullying for girls. I don't know of any for boys, but maybe the girl books would give you some suggestions? American Girl Magazine and Discovery Girls are other places to look.
Also, Family Circle magazine has excellent articles on things like bullying (they recently had a feature on hazing) and a regular column on children's problems, so you might see if you can find anything on their website.
Sorry I didn't just come up with a word for you, but it did lead to an interesting discussion with my kids!