Your best bet is to post this question on johnbridge.com. That website is dedicated to tiling. If you think THIS forum is great, you'll LOVE that one for your tiling questions.
One thing to think about: sure you want to do green board? Have you considered Schluter Kerdi? It's a waterproofing membrane that goes over reg drywall and it gets rave reviews from users.
As far as tile size: in general, smaller tiles (6x6 or smaller) will adhere easier than larger ones on a ceiling. Besides that, consider what fits easiest with minimal cuts, and what looks good to you.