There are several programs out there that will print posters, the one i use is actaully called "poster printery"
you can get it here.
http://www.bluechillies.com/details/568.htmlI like this one becuase you can set the size and see how many sheets you will use.
You can make great side art with this and good photo paper, and some skill in cutting. I used this for my control panel overlay (which is under plexi) and it turned out excellent. I also used it for my Marque, but i agree, i cut them and actually glued the edges of the paper together ( not overlapped, that actual little paper thin edge, to another paper thin edge) and you can still see the line where they are glued, banner paper is in the future. but for overlays, or sideart, with a little bit of skill it could look great.
Clok