Well, I actually meant 300x300! You are right, 300x300 doesn't exist & I was aware of that. That is why I made a suggestion so that 300x300 fits perfectly. Yes, a 300x250 can also do the job, but that would mean that there would be a gap of 50 pixels between the ads & I wanted to avoid that. That way, there is no empty space & at the same time, 2 advertisers get to show their ads. The ads will differ in their look & feel, so it would be apparent that they are 2 different ads.
I also suggested that because that will open new door to the new ad format of 300x300. Look at it, it's square. We all like an even square, don't we? That would mean advertisers get one more opportunity to display more of an ad (although 50pixels may not sound like a lot). But you can't beat the even-ness of the Ad. A very good example of usage would be the Sidebar in a WordPress theme. Most of the layouts that we do for our clients have a sidebar that is about 300px wide & having a perfect square block would look awesome!
If implementing 300x300 would take time, then I think a 300x250 ad could replace it. And Jax, thanks for supporting my idea.