What size image do i need to do this justice?

I always suggest full size or very close to it. As you enlarge a photo, the dpi starts to drop. Let's say you had a 300 dpi photo that is a 2" square. By the time you make that as large as you want it to be, it will look really bad. On the other hand, if you had a full size photo that looks really good to you and it was only 100 dpi, it would still be a usable photo since you don't have to try to enlarge it any further. I'm talking about the actual printed dimensions of the photo of course. Not the size it might look on a computer screen.

300 dpi at what size?.....300 dpi at 12 x 18 you are good to go......300 dpi at 2 x 2 you are not.....

Thanks for getting back to me.

In reference to the link below, what size could i get away with then?

Sensual couple stock photo 27745977 - iStock

The largest size they have.....For "re-sale" you need an "extended licence"....

Well, it's nice to have the largest size they have, but what is the size you need? That image is a square with not too much wiggle room for cropping the sides of the image. So you're kind of restricted to how wide you can go on the shirt. Choose that size for your image.