What Resolution to draw sprites for Mobile game??

Common question, but a convoluted one. :) A single pixel in Unity has no fixed size. If you have a character that is 16p x 16p pixel resolution, she can take up any amount of world space that she wants via Transform scaling. You may ask, "but how many pixels will this take up on-screen?", well, that's where "camera space" comes in. You never have to worry about exact resolution of a screen or display.

All you worry about is how much the player can "see" of the world space, and you do this by scaling the camera viewport. Your camera can render 10000mx10000m in the game (you couldn't even see a 1mx2m character), but that display will still render 1920x1080 resolution. And vice versa, 1mx1m camera viewport is still 1920x1080 on the display. You can only control how much your camera can see, and this changes the "size" of a sprite on camera (in camera space), which in turn changes the exact pixels of the sprite.

And likewise, if you have your 10km camera rendering 1mx2m characters, you can scale those characters up so the camera can see them. You can make your 2m tall character 5kmx10km tall. Now she takes up 1/4th of the screen. In other words, the pixels don't matter. Only the world space and how much of that you camera can see matters.

So, if you need sprite sizes, you say "32x32 characters, bushes should be half that size, and ogres should be twice that size" and be done with it. Because after that, you are simply adjusting the camera and scaling your sprites so that they look good on the camera.

(Unity has a package for nice pixel cameras here, also make sure you choose the right filtering for your sprite images - should be point filtering - otherwise scaling the image up will cause blur)