Game Size for publishing on Websites?

Hi,

Quick Unity newbie question. I'm making a 2D point & click game to publish on game websites (websites like wooglie.com for ex.). Is there a recommended size that I should make my game in for websites like these?

And how does resizing your game work exactly?
Right now my game is set on 640x480 in the bottom window in Unity. However, I can't open my build in 640x480, because it's cutting off the borders. I have to open my game in 1024x768 for it to display the entire image.

Does anybody know why that is?