Maximum or optimum printing size for an image

The file is just pixels. The print size is determined by whatever pixels per inch number you assign. It's basically self-explanatory: pixels per inch. It means exactly what it says.

Ppi is a measure of pixel density on paper. Obviously, you need a certain minimum pixel density for the print to look good. For an inkjet print, 240 is usually sufficient.

For commercial offset print, the standard figure is 300 - but this is somewhat arbitrary and it won't be "unsharp" at lower ppi (down to perhaps 200 or so).

Here's the thing: all these figures are based on a small print to be viewed from less than arm's length distance. Bigger prints will be seen from farther away, so ppi requirement drops accordingly. A large wall-sized banner can be printed at 15 ppi and look just as crisp and sharp.

So the bottom line is that a good file will more or less work for any print size. 6000 x 4000 pixels is standard sensor resolution these days and can be used for practically anything.

The really important thing is that the file is of good photographic quality! Sharp focus and no movement or camera shake, and optimally sharpened for the intended purpose.