tricep pullovers with dumbells,similar to french press as regards movement,these have given my tri's some real mass,keep em strict and heavy.
as far as arm size,ithink its been mentioned before,adding 15 lbs lean body mass will increase your armsize an inch,all i can say is that my training partner and i witnessed this.18" is the magic number im chasin,it all depends on b/f as well and your overall genetic max and body shape.