Best practices for a 2D tile based mobile game

I'm in the early stages of building a 2D game with Unity that targets iOS and Android. I have a couple of concepts that are confusing me and I could use some help to clear them up.

Basically, the game is as follows. (Game is played in portrait only)

• The user controls an object (a sprite) and is able to drag them left and right.

• Obstacles will be spawned from the top of the screen, and fall towards the player.

I'd like there to be a fixed number of spaces that the obstacles should spawn in (horizontally) and however many can fit vertically. Let's say I want 10 spaces horizontally for an example.

Are there any problems with my thinking? What happens on phones with different resolutions, or a tablet for instance? Would the spaces just stretch to fit the screen, each taking up 10% of the width? Would the sprites just be stretched to fit the size of the space?

Sorry if this question is confusing, but I'm having trouble trying to visualize this.