I'd second socialgiraffe and soulclaimed - quickly measure from the collar if you're unsure, but doing it by eye is probably accurate enough unless anyone chases it up with a ruler afterwards...

Hi, I'm Simon - I've been skimming information from this wonderful forum for long enough that I thought it was time I attempt to contribute something useful!

Anything with enough polyester will work fine - I've had good results printing on coloured 50/50 polycottons - obviously you can only die the t-shirt darker than it is so we we're just doing black prints. There seemed to be enough polyester content in a 50/50 to get a pretty solid black tone.

If you hate the feeling of poly on skin, I've had great results on American Apparel dri release shirts. They're an 85/15 polycotton so they don't feel anywhere near as synthetic as other sublimation T's I've used. I know they're pretty pricey though...