You find your niche by racing. Various types of races and see where your skills lie. It is useless to point to a physique because, for example, there are great sprinters that are not big at all. Look at McEwen or Fabio Baldato. They are not big men. Lance isn't particularly small but he's a mountain goat. Simoni is rather small and he is a fast time trialer. Cassagrande is small and looks like he would spin up mountains but he mashes gears as big as Ullrich. Body type means very little to what type of rider you are. That is something you need to find out on the road.
Originally posted by crucifixion12
I'm a new rider too. How does a rider find their niche? Is it by trial and error, or do you say "hey, I'm built like the guys who are sprinters, I'll train to be a sprinter as well"? There is a lot of very good and interesting info in this post, I've learned a lot I didn't know by reading this.