Apple made billboard advertisements using iPhone images. Even for phones with a mere 8 mp, I'd expect lens sharpness to be the limiting factor. You'll have to photograph a resolution chart with your phone and adjust as needed.

The real answer is likely associated with the angular resolution of the human eye. But in practice, about 200-300 ppi is sufficient for viewing pixel-peeping sharp images handheld. Since about 60lpmm on crop sensor (24mm x 16mm) at 6000x4000 is reasonably sharp, a 30in x 20in print should be fine for viewing about a cubit away. You can use the above values (or whatever you find acceptable) as a basis to calculate the print size for different viewing distances, image resolutions, and lens sharpness. The weakest parameter is the limiting factor.

• 60-65 lpmm is sharp enough for me on crop sensor. You may require more or less. For full frame, 45 lpmm may be acceptable. For even smaller sensors, much higher may be necessary.

• For a more detailed explanation, see this answer to Is there a general formula for image size vs. print size?

As for artifacts:

• Compression artifacts stay about constant at the same print resolution because they're pretty much limited to 16x16 blocks.

• Sensor noise depends on light levels, ISO setting, and other factors. Assuming a clean image in good light, it can pretty much be ignored. Otherwise, treat similarly to lower resolution images taken in poor lighting.

• Similarly for dynamic range and lens characteristics, such as sharpness and contrast.