Seatpost length is usually measured from the saddle rails to the end of the post. Here is Thomson's dimensioned drawing and you want to match the "L" dimension with you current post.
http://bikethomson.com/blog/wp-conte...-Dims_revA.pdf
You could, of course, buy a longer post than your current one with no problems unless it's so long it hits one of the water bottle braze-ons in the seat tube. Even then, just get out the hacksaw and trim off the excess.