We look at it at a distance. This distance will increase with the size of the picture, and will typically be 1,5 times the diagonal.
Because of this, there really is no practical limit to how big a picture can be reproduced.
This is why it’s at all possible to have enormous posters filling the entire side of buildings. You don’t look at them at point blank range.
The resolution of the picture will be sufficient whenever it’s impossible for the human eye to distinguish individual pixels.
Any picture that is printed with at least 180 ppi resolution will be viewable at a distance of 0,5 meters without the individual pixels being discernible.
This means you can hold a picture measuring 75 x 50 cm in front of you, and as long as the resolution is approx. 5300 x 3500 pixels (18.55 Mpx) you can’t see any individual pixels at this range. When viewing this picture at the more normal distance of 135 cm, the resolution could in fact be as low as 65 ppi (2.5 Mpx) without any individual pixels standing out to the naked eye.
Keep the printouts at 180 ppi or better, and they can be studied at arms length without any pixels standing out.
Remember that there is a difference between the printers resolution in dpi (dots per inch) and the resulting resolution of the picture in ppi (pixels per inch).
A modern printer typically has a resolution of 1200-2800 dpi or more, which is used to make the raster required to mix all the colors needed from maybe less than a dozen different ink cartridges.
The useful maximum resolution of the printout can be found by dividing the dpi of the printer by 2.5, which means that a printer with 1200 dpi can create a printout at 480 ppi, and THAT can be scrutinized as close as 18 cm without showing any pixels.