Take it to your LBS. They should have either a set of calipers (nice ones) or a stepped seat-tube measuring... thing. Either way, your LBS should be able to tell you what kind you need. While you're there, cough up the 15 bucks for a cheap Kalloy post. They come in just able any size you'll need, are light enough (esp for a conversion), adjust fine, and are cheap. All you need for a conversion.