You should really re-think your stance on the size. As a website publisher, anyone wanting to run a non-standard size is going to have to A: pay me for an exclusive spot, because I can't run any other ads in that slot and B: pay for my time to write custom code for your ad. Doesn't happen often.

That's all assuming that's a size I can even fit into my design without making the site look bad. The 250px width is awkward. For us, our right rail is sized around a 300px wide big box ad, 250 is enough off of that that it's going to ruin the solid right-side margin we've designed.