Hey 
for non-poster products, Spreadshirt says the maximum is 4,000 x 4,000 pixels and 10 MB. However, now I have read several times that this is outdated. One can also upload larger images. Now of course the question is, what is the “real” maximum? I would like to create my images as large as possible and then scale them down in the editor. What is your guideline?
With kind regards