If I use an image for the background, does it matter if I make a really big (vertical wise) background. Like, does the background take up bandwidth just once for the picture, or does it eat up more bandwidth depending on how long the page is because the bg image has to keep duplicating itself? For the 56k users, will the page load faster if the image size is smaller? Sorry, hope that made sense.

Yes, the smaller the BG is, the quicker it will probably load. And I don't think that they take up more bandwith as they get bigger, but don't take my word for it because I'm not completely sure (almost, though).

Well, you have to find the happy medium. If the image is too big itself, it will take time to load. But if it's too small, then it will take time to duplicate itself so many times. Hope this helped!