Google's Picasa software - has an excellent collage feature in it - it will allow as many photos as you want, has auto organise and organise yourself options. It comes with lots of paper sizes and you can define your own. I would use that and then save it as a jpg and upload to get printed. I never use any of the photo sites collage feature because I think is better and cheaper to create collage yourself and get printed as one file. Picasa 3.9 is the current version.