I wouldnt worry if i were you. There are actually pro MMA fighters your size and even some that are shorter or lighter than you in some organizations, such as WEC (owned by the same people who own UFC but WEC focuses on the lighter weight classes).
Don't worry about rolling with people larger than you, it's about learning not about dominating. But if you gain some size some day (you're only 17) or if you just eventually progress to the point where you can hang with most people who are a bit bigger than you .. then you should be even better off when you are put against people your size.
Just my 2 cents
Good luck, and have fun