"Buy Credit" is done by a fixed amount like $5, $10, $25 ..all the way to $1000. The larger amount the bigger cash bonus - up to 25%.
The $ amount now on your account can be used on any of the 5 banner sizes available.
CPM prices right now is based on area covered (pixels) by the banner, and might be adjusted if average CTR does not reflect the larger size. Right now prices are;
Button - 120x60 - $0.19 CPM
Standard - 468x60 - $0.76 CPM
Leaderboard - 728x90 - $1.77 CPM
Skyscraper - 120x600 - $1.95 CPM
Rectangle - 300x250 - $2.03 CPM
Above prices is the same no matter what targeting you chose and should be compared to full targeting on category, time and geographical target.
As an advertiser (or exchange member) you can use your dollar amount on any size. You can even make your own refund from one banner size if you decide to use your credit on another banner size.