1024 x 768.

I can tell you that without looking at the poll because that's the smallest size that most people have their monitors set to these days. You will still have some viewing at 800 x 600, but not too many. Anything larger than 1024 x 768 and you're cutting out a signifigant proportion of the internet viewing population.

While most people on here probably have monitor resolutions set higher than 1024 x 768; that is the exception, not the norm.