If standard IAB banner sizes aren't right for your widget then use text ads. CPM is always the last choice. Regardless of CPM, CPC or heck even CPA, test your

If standard IAB banner sizes aren't right for your widget then use text ads. CPM is always the last choice. Regardless of CPM, CPC or heck even CPA, test your traffic using private run of network ads. You'll have complete control over them so you can see the honest to goodness truth and be able to relay that to the ad network sales folks for initial measurements and pre-sales.