Recommended File Size for Background Images

I would like to use background images for the main layout background, center and sidebars. I am concerned that large background images will slow down the site time. Is there a recommended file size for these images?

--edited: I have now minimized my image quality as much as possible to speed up the load time, but it is still too slow. Is there a way to discover what is causing the slow load time? could it be my background images, which aren't more than 200kb in total?

Additionally, can I stretch an image to fill to the bottom instead of repeating the image?

I would also like to create an image for the center layout which is no wider than the sidebars. My current center image is the full width of the layout. I would like the image to still remain fluid, but no wider than the inside edge of the sidebars. Is that possible?