I would probably build the collage print using Photoshop. It good also be done with Photoshop Elements or some other external editing program. I would create my background image and size it appropriately and then arrange the images on that background. Then I would save it as a JPEG and send it to a lab.
Thanks Jim! I actually just found how I can create custom sheet sizes in my XPS Document Printer driver. So, now I'm all set. Thanks so much for the response, I appreciate it!