Notes
Printful has no published rejection line — what that means for you
Sellers read "it uploaded fine" as "it is fine". On Printful that inference is not available to you, because there is no documented line to have crossed.
What is documented
At least 150 DPI across the print area, with 300 DPI as the working target. Files cap at 200 MB and 20,000 pixels on a side. Requirements beyond that are set per product and published on each product page's File Guidelines tab — which is also why a single "Printful size" is always a simplification of one particular product.
What is not documented
A density below which the upload is refused. You will find confident claims of a hard floor in third-party guides; they are not in Printful's own documentation. Treating an invented threshold as real is worse than having none, because it tells sellers that anything above it is safe.
So what do you check instead
Effective density across the actual print area — pixels divided by print-area inches, not the DPI tag written into the file. Then check the headroom: how much wider the artwork can be printed before it drops under 150 DPI, because the size run will consume some of that. And check the product page, because the print area you are measuring against is set there, not platform-wide.
Frequently asked questions
Does that mean Printful will print anything?
It means the upload step is not where quality is enforced. The consequence of a low-resolution file shows up in the finished product rather than in an error message, which is exactly why the check belongs before the upload.
Where is the authoritative number for my product?
The File Guidelines tab on that product's catalogue page. Print areas differ between products, and the same file therefore has a different effective density on each.