I think its the Banner Quality that really matters. Recently I saw an Amazon banner after seeing which I was like "Wow". It had really cool graphics, and displayed the 10% discount and 3 Top Seller Books perfectly. Can't remember where I saw it.
But, I think if the banner has a "Wow" factor to it, it can help you make a sale.
So, I don't vote for a size, I vote for the "Wow" factor in a banner.
