ofnuts wrote:
No need to overdo it.
This shop recommends 200DPI mini, 300DPI optimal. 1200DPI would yield an unwieldy 10000x10000 picture (likely above the max uploadable size).
The reason I recommended 300DPI minimum and higher if possible is because most of those online printing shops assume you are using SVG graphics, not rasterized formats. I run several different shops on cafepress and spreadshirt and from my own personal experience the rule of thumb is "the smaller the image, the higher the DPI." For printing one should always pack the most information possible into the graphic due to things like fabric construction and bleed area.
As you pointed out, one is always limited by the limitations of the upload file.