Here's some data that might help. We run a lot of non-standard ad sizes and you'll see how they compare against the "standard" sizes..

100x133 Image + Title 4%
110x150 + Title 4%
120x240 Image 3%
120x60 Image + Title 5%
120x90 Image + Title 7%
125x125 Image + Title 7%
140x140 Image + Title 2%
140x140 "Bloggystyle" Long Text 2%
150x100 Image + Title + Text 2%
150x150 Image + Title 7%
150x175 Image + Title 2%
175x100 Image + Title 3%
190x140 Image 9%
200x100 Image + Title 3%
200x100 Image + Title + Text 3%
200x300 Image 0%
250x150 Image + Title 2%
250x200 Image 0%
250x250 Image 2%
300x250 Image + Title + Text 8%
66x66 Image + Title + Text 0%
90x120 Image + Title + Text 0%
468x60 Banner 1%
468x80 Banner 1%
Custom Ad Size (do not use) 0%
728x90 Leaderboard 6%
160x600 Skyscraper 1%
Text (Contextual) 7%