What decides the largest size
A bitmap holds a fixed number of pixels. Printing it larger does not create more of them, it just spreads the ones you have over more paper. The measure of how thinly they are spread is dots per inch, and it is the only number that matters here.
Divide the pixel width by the dpi you want and you have the printable width in inches. An image 3000 pixels wide at 300 dpi gives 10 inches, or 254 mm. The same image stretched to 500 mm is running at about 152 dpi, which is fine on a poster and visibly soft on a brochure.
How much resolution you actually need
The usual advice is 300 dpi for everything, which is right for things held in the hand and wasteful for things seen from across a room. Viewing distance is the variable nobody mentions.
- 300 dpi and above. Anything read at arm's length: business cards, brochures, packaging, book pages, photo prints.
- 200 to 300 dpi. Still good for most printed matter. The difference from 300 is hard to see without comparing side by side.
- 150 to 200 dpi. Fine for posters and exhibition panels viewed from a metre or more. Standard for large format work.
- 100 to 150 dpi. Suitable for banners and signage seen from several metres. Close up it will look soft.
- Below 100 dpi. Too soft for almost any printed use. This is usually a sign the file came off a website rather than from the original artwork.
Why the file size in megabytes tells you nothing
A heavily compressed JPEG and a lightly compressed one can hold identical pixel counts and print identically, while differing tenfold on disk. A PNG of a flat colour logo can be tiny and still be 4000 pixels across. Megabytes measure how efficiently the file was packed, not how much detail it holds. Pixels are the measurement, which is why the tool above reports those first.
Vector artwork has no maximum size
If the artwork is stored as drawing instructions rather than pixels, none of this applies. There is no resolution to run out of, and the same file prints correctly on a business card and on the side of a building. That is why printers ask for vector logos rather than large ones.
If you drop a vector file into the tool above it will tell you exactly that rather than invent a limit. To find out whether the file you have is genuinely vector, which is a surprisingly common thing to be wrong about, use the vector check.
Enlarging an image that is too small
Resampling upward in an editor adds pixels by guessing at what would have been between the existing ones. It softens detail rather than recovering it, and on lettering it tends to show badly. Modern machine learning upscalers guess better, but they are still guessing, and on a logo they will quietly reshape the letterforms.
When the artwork is a logo, the real fix is to get the vector original from whoever designed it. When it is a photograph, the real fix is the original from the camera rather than the copy that has been through a website or a messaging app, both of which resize aggressively.
Is my file uploaded?
No. The dimensions are read from the file header in your browser and the file never leaves your machine. Nothing is sent, because the page makes no external request at all.