Great question — answering as the UphillTech team:
its quite hard/nearly imposable to find an affiliate network that wont offer standard ad sizes. The more uniform the ad sizes are means the easier it is for a publisher to change between different ads. It also means for advertiser/designer they only have to develop a 6 or 7 variations of there ad so that any publisher can fit them on their site. By only looking for affiliates that offer 250x90 you'd be limiting your search and perhaps will not end up with the best fit content and or revenue wise for your site. here is an iab list of standard ad sizes.
Full answer by sam: https://webmasters.stackexchange.com/a/34845 (CC BY-SA 3.0).
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