hey welcome to the forum!
230's perform best with 20" rears. and personally, i like 13/41 gearing but that may be a little tall for some peoples likings. what kind of riding do you do?
and 230s need very special attention in the chain department. buy O-RING chain, i use standard EK 520. i get my chains from
MOTO-CHAINS , Motorcycle ATV Motocross Chains n Tires pretty good prices.
for sprockets, steel is the way to go. aluminum doesnt last long and should really only be used on all-out racing applications. i prefer sunstar myself.
if you are replacing your chain and sprockets because the chain keeps coming off, you probably need swingarm bearings, too. very common on these bikes.
i suggest you look at the monster 230 thread also.
good luck!