I would like to print a Polaroid like image that is 336x407 mm big. What size is that in pixel and what PPI should I use?
You have to know the pixels in order to get the pixels-per-inch conversion. For example, if you have a 100x100px image that you wanted to print as 1x1in, you would print at 100ppi. You don't have enough information to translate from measurements to pixels.