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.
A pixel does not have a size until it is converted into a physical form by printing it on paper, displaying it on a screen at a specific resolution, or making it into a card for spectators to hold up at a football game, for example.
The most important thing to know about pixels in digital imaging is that a pixels is simply a "spot." It is generally (but not always) square. And it is always one solid color. It is one discrete piece of digital color information, nothing more. A pixel has no size.
A millimeter is a physical size that does not change.