If the frame has a 20-30 tubing label on it, good bet is that it's 1" (25.4 mm). But if you're in any doubt, and don't have a caliper, take it to the LBS and they can tell you. The opening is usually just a couple tenths of a millimeter larger than the post size. A greased one should still take some hand effort to get in. I won't make any anal(-)ogies.