Well, pixels per inch come into play when you're going to print something, for example. In games, however, ppi doesn't do anything at all, width and height is what you should care about.

First of all it's important to use sizes that are a power of two (x and y doesn't have to be the same, though). This has something to do with how textures are stored in video memory. Other sizes work aswell but you can cause up to 75% of wasted vram for your textures.

As for the maximum size, there isn't really an upper limit (every graphics card has a limit, of course, but it's usually pretty high).
Limit-wise, you can easily use textures like 4096² but this is already pretty huge and usually not needed unless you're trying to do something special (like atlas textures, for example).

I'd just recommend you to use texture sizes that you think are good enough to contain the detail that you need while trying to keep them as small as possible in order to save memory.