The leaderboard is the large banners you see at most pages and they are 728x90. 300x250 is the medium rectangle that has been popular in the last year. Many adult sites work with odd-shaped banners that are half the size of the leaderboard or 468x60. Along those lines, the skyscraper (which is the 120x600) has been modified and displayed as a 160x600 a lot. The thinking is that sizes that are diff from what everyone else runs will draw more attention/clicks.
Here's the url for the Internet Advertising Bureau (
http://www.iab.net/standards/adunits.asp ) where you can find the "standard" sizes.
-AdultAds