It's gonna have to be huge as a 12"x12" 150 dpi render[average t-shirt size and print res] is going to be 1800x1800 pivels, and if you want to go for real hi quality I often doube to 3600x3600 and reduce in PhotoShop for print final image. With full everything if you have the power/time....LOL...seems excessive but is often worth the wait on a looooong render.

 

Once in a while I look around,
I see a sound
and try to write it down
Sometimes they come out very soft
Tinkling light sound
The Sun comes up again