I would like to make poster-sized images of a few of my students photos to display in the classroom. I'm familiar with http://www.shutterfly.com and http://www.kodak.com but haven't processed images that large. I was wondering if anyone has recommendations.
What you need to be careful of is the resolution of the picture. Even if the resolution is fairly high quality, it may still not be good enough when you enlarge it to something poster sized. I am not sure how big of a poster you are making, but this is definitely something to take into account.
I have not used any of these services myself, but have made posters myself by printing a photo to span multiple pages. The only trouble I've had with this approach is that I then have to tape or glue all the pages together and make sure they line up perfectly both horizontally and vertically.
Have you considered BlockPosters.com? You upload your picture and it sets it up to span multiple pages. If you have a color printer, it can actually turn out fairly nicely.