The current iPhone (non-plus models) have a screen resolution of 1334x750px. If I were to use one of these in a design, what physical printed size could I scale the screenshot to where the resolution would still make an acceptable print?
If you were to print it at 300dpi, which is the resolution usually employed for quality print work in publications, then simply divide the each pixel dimension by 300, to give you the size in inches.
That would be 4.45" x 2.5"
I suppose you could perhaps get away with 200dpi, which would make the image a little bigger, but slightly less quality.