The idea here is you download the image and print in the size you want
then you don't have to wait for the actual print to arrive to your place, or it gets lost in the way, or being damaged by the mailing system, you can't print when you want, but don't be an asshole sharing for free with your friends, the income is used to afford to spend time creating new stuff and selling at a reasonable price.
 
The images are mostly a3 paper size, 300 dpi.