Hi Jane,
If you didn't get any large printing service information yet, you might try this Bask Shop (
http://www.bask1.com). They have several branches in Bangkok but for large printing they will send it to their headquarter, located in Vibhavadee road (I usually went there to have my poster printed).
I usually use their service to print my poster design, and as long as you give them a good, hi-res photos, then they will print it nicely. They are using HP brand printing device, and they accept printing until 2m width of paper.
For printing, they charge around THB 550 / meter square of paper, including lamination. Have several selection of paper (matte, glossy, etc). Quite Quick, I print 10 copies of 1x1.5m poster, and they can finish it within a day. Problems: not all staff can speak English.
Agus