Why not just list DPI at the outset?
By @rss412
Because the DPI (in reality it's PPI, if you understand digital images and the printing process) is irrelevant. What is relevant is the image in pixels (which is shown) and the size in inches/cm you want to reproduce this asset (you know this). A small calculation will bring you to a required size in pixels for your image.
Do you want an example? An A3 (29.7cm x 42cm; 11.693in x 16.537in) picture at 300ppi has 3508px x 4961px. An A5 (14.8cm x 21 cm; 5.827in x 8.267in) picture at 300ppi has 1748px x 2480px. The same A3 picture at 150ppi has 1748px x 2480px, which is the same as an A5 image at 300ppi.
So, I can set the ppi value to any value I want, as long as I do not change the pixel value, I have changed nothing whatsoever.
If you are selecting for print, look at the required ppi (set it equal to the required dpi, even that it is not correct, if your printer gives you a dpi value) and the size you need to reproduce, and when the picture is somewhere above 200ppi you should be good for print. My printer gives a warning for images less of 240ppi for a high-quality print. With a program like Photoshop, I can easily upscale a picture file up to 200% and with carefully sharpening, I will get a near to perfect print. With some tricks, I can improve the picture quality (if it was lower) or even go higher (if the picture quality is best).
Incidentally: a picture can always be downscaled, without any loss of quality.