Steve, that will depend mostly on the phone camera’s resolution. My iPhone 11 captures 4032x3024 images. Printed at 300 dots per inch this suggests a print no larger than 10x13 from an uncropped photo. At 300 dpi you can pretty much hold the picture right up to your face without seeing pixelation.

I’ve personally printed up to 11 inches on the long side in photo books from a phone photo without any trouble. I’ve not tried larger than that, not because of any resolution issues but because I haven’t needed to.

You can probably print larger than 10x13 if the viewing space for the photo is sufficiently large. I wouldn’t do that in a book because they’re typically held in a lap or on a table less than two feet from your face. But on a wall people will just stand back a little farther to look at large pictures at lower dpi levels and they’ll look fine.

You can also upscale photos using popular software programs to add resolution. It won’t be perfect because the software is guessing what the detail looks like, but the results I’ve seen from several software packages range from good to impressive.

Other smartphone cameras can have larger or smaller resolution, so you’ll need to look at the specific camera image to know for certain how large it will print.

Also note that the image resolution can be misleading regarding how much detail is actually captured in the photo. Night shots, for example, may have less apparent resolution as the phone combines several photos into one image. If there’s any motion of the camera or in the subject matter the results will not be crisp. But these photos can have their own charm.