http://www.juicyads.com/campaigns_request.php
look on the right under "Available Inventory", shows ya what sizes webmasters have available currently through
Juicy Ads so the top sizes in that list are more popular.
you could still do more since the site is new, don't wanna not have something for a rare few.
728x90
468x60
300x250
160x600
125x125
120x600
120x60
etc... but I agree 300x250 is the one I use 99% of the time.