As a designer who has worked with custom shirt prints let me give you some advice. For those of you who are submitting designs, make sure your design is as large as possible, preferably in the 2000px x 2000px range. Why? The quality of the print. If they have to blow up an image, it will lose it's image quality and start to become distorted, pixelated, fuzzy, blurry etc. By scaling the image down you will retain your designs quality, and this also helps when printing on multiple sized shirts. Bigger shirts means the design needs to be bigger to be proportionate, make sense? Another thing, color. The more color you add, the more expensive it will be to print, if money is an issue. Also a lot of the times the printing service will want the individual colors in a design to be in separate layers in your .psd file.