The quality will of course depend on the quality of your images. When I used to do a lot of trade show work I dealt with this place
http://www.bigposters.comThey are on Long Island USA. Depending n the medium you wish to print on, remember that such a project can be quite costly.
Notes on Bigposters - they actually answer their phones if you call there, and they are small enough to keep on top of their jobs yet large enough for quick turnaround. Have been using them since about 1998. At that time, they even cut me a small price break for a charity project which was appreciated by the organization.