I imagine you would want to examine your stories and see which puppet could be added to one of your stories. My wife says that since I am a tall person I need bigger puppets. She says the little ones tend to get lost next to me. (Probably hiding behind my stomach from the side view!) But I have seen someone use small or large and to me, they both look good.
You may want to go with a puppet you can change clothes for, so he/she could be in several different stories. That would probably be a boy puppet. If you have a collection of hats I suggest a larger puppet that could wear a different hat based on it's character, such as the bear.
Depending on the instrument(s) you play you might want a puppet that can help play the instrument.