If your testicle is an odd size, you should immediatly seeks medical attention. They need to be even sized. (and in metric measurements). So a 4cm ball is normal because it is even, but a 3cm one is not because it is odd.
Based on what I learned from the movie Van Wilder, if you don't masturbate, your balls will grow to huge/humoungous size. (at least for dogs :-). So by masturbating lots, you may be able to shrink your balls from odd down to an even size :-)
Seriously: it is perfectly normal to have slightly different size, it is perfectly normal to have one hang lower than the other. If you feel the difference is truly abnormal, then only a qualified urologist could gauge whether the size difference is normal or the sign of something wrong with one of them.