the one that comes stock on the clockwork is like 60mm. it's godawful looking too, so you should be able to get it for basically free.

ideally youd get a smaller frame and have a lot of seatpost, but whatever works.

EDIT: looked on bikeisland to see if they sell that weird silver stem on the cheap (they don't) but I saw this stem, apparently made for people who want to make wrong sized frames work for them:
http://bikeisland.com/cgi-bin/BKTK_S...ls&ProdID=1436

can be ordered from sizes ranging from ~2 inches (60mm) to three feet (909mm)

and that's so not a typo