I'm making a fixed background for my myspace, what would be the best size (in pixels) to use?

Too small and the image will be pixelated, too big and it will be cut off at the edges.

I'm making a fixed background for my myspace, what would be the best size (in pixels) to use?

If your using a fixed image then don't go above 750 pixels wide.

This means that anyone with 800x600 resolution will see it fine.

Or the size it looks best is probably the best size :)