Questions about texture compression

So, when I choose the texture in the Inspector window, three tabs appear: Default, Autonomous and Android. I can choose some variants to compress this texture, but:

  • My texture is a png (so it's RGBA). When I click on the default format -> I can see things like Alpha 8, R 8, RGB 24 bit. So if I choose R8for example, does that mean that my GBA the channels will be thrown?
  • What is the Max size? My texture is 600×375. If I choose 512 it will cut my texture to 512×375 or what? And why is it called Max Size if I work with a single texture right now?
  • If I choose Default-> Format for Automatic and does not make any substitutions on a specific platform Will Unity choose the default texture compression formats for a specific platform using this table?