Well size doesn't matter as much as thick/thinness when it comes to jazz. Plenty of jazz guys play bigger crashes, plenty play smaller crashes, the common denominator though is that most are thin because for jazz you want something very responsive even to the lightest touch.
Size is a matter of preference. Small crashes are going to be quick, punchy, and have a lower volume cap. Bigger ones will have larger washes, give you a brouder sound potential, and can also be ride-ed.
Thanks for all the advice. I wound up buying a Sabian 18" off Ebay. So I now have Sabian 14" high hats, Sabian 20" ride and now the Sabian 18" crash. I guess now I will have to start thinking of a Splash to add. Nobody told me that adding to my kit was going to be so addictive! Thanks once again for the collective help.