HI
I would like to know, which textures sizes and resolutions are best to use with each platforms? (On PC, mobile, consoles)
Is there a way in unity to limit those resolutions for a certain platform?
Thx. 
The best way, is to look into (google/research) what the target console/mobile native resolution is, for X-Box, PS, mobile, etc. Then you can better narrow down you final resolution.
Yes, in the import settings you can set the resolution that the texture will be in the build. That setting has a separate tab for each platform that you have installed, so you can choose different values for each.
There’s not going to be a one-size-fits-all number, though. It’s always going to depend on how big a texture is stretched over the screen, and you could have multiple texture sizes in a single project. You have to eye-ball it, more or less.