Measure the one you've got and get the same length. Basically, though, Road posts are 250mm and Mountain posts are 350. You should never have to cut a seat post; they are not like fork steerer tubes that you cut to accomdate the HS. The most important size in a seat post is the Diameter. Your old seat post should have the size embossed on it near the bottom. If not, you'll need to measure it in MilliMeters.