A good rule of thumb is less than 5k. Background images that are too large can upset even the most avid web surfer. Also, keep in mind that when you view your page, the graphics are already cached on your system. Many developers see the graphics quite quickly and are amazed when people complain. Be sure to empty your cache and then test the speed of your page.
Many people believe that the only way to make an image smaller is to scale the dimensions down some.
With GIF images you have another option. A 4 bit gif is usually quite small. So bring the dimensions down a little and reduce the colors. Interlaced images take longer to download than non-interlaced images. Interlacing your background image does nothing more than irritate the surfer in most cases.