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?

Stretching background images is not possible with CSS, you can only fake it with foreground images that are put under the other content with the help of the z-index property: http://webdesign.about.com/od/css3/f/blfaqbgsize.htm

200 kByte is not excessive but still fairly large. I'd cut them down some more if it's still too slow.