A couple tenths of a millimeter it sounds like. Just buy a new post, turning down a aluminum seatpost is going to be more expensive than a replacement.

The minimum for posts nowadays is 4 inches or 100mm but that doesn't really tell the whole story since seattubes often extend above the toptube. I like 100mm but also making sure the bottom of the post is at least 50mm under the lowest welded joint at the seattube/toptube/seatstay junction.