I'd steer clear of the 250x100 and other odd sizes. Stick with IAB sizes 160x600, 300x250, 728x90, which are the predominate adsizes. You'll be more likely to get into rotation on websites as a default ad with these size.

Also on corn_41 edit2: you're going to hear the word transparency a lot in the digital ad space. Everyone wants it and expects it so the more information you can put out the better.