I would like to ask webmasters, what optimal website page sizes are?

for what?

I think developing sites for 1024*768 pixels (around 700 pixel in width) is a good way. For discussion boards or other sites containing much text elements a variable layout may be preferred.

It used to be that bigger was better and larger monitors and pixel resolutions meant that you should of larger. However, the web is evolving and a lot of focus is now on making sites look good on half-held devices as well.

"half-held devices " doesn't sound like they will last long so i wouldn't worry about those

It used to be 800x600 was the old standard, and many people still think so. However, I think it's safe to say 1024x768 is certainly the way to go. I mean, put your computer resolution down to 800x600 and you will see how many sites aren't built for that resolution. Youtube, MySpace, etc... (correct me if I'm wrong on those two, those are just my results).