doesnt make a difference atall.
im a heavyweight and im 5ft 11 and ive won all but 2 of my fights by knockout. (i got beaten in one, and won on points in the other) and i have had 11 fights

youll find that if your a light division, most of the people will be that height aswell, so dont worry about it.
i got a guy in my club, ben, he stands at 5ft 5 and could ko me in a flash.
being short means you have a huge advantage when you get close, long arms are infact a problem when you get close, as you cant produce as much power.

made sure you got good footwork, and a sound defense.
then slip under jabs and take em out!