Question How to chose a sprite dimension for a certain resolution?

I am planning to create a game that will run in 1280x720 windowed exclusively, just for pc and without the possibility to change the resolution. I am extremely new in unity so I dont really know how to set things up, any help would be extremely appreciated.
How do I setup unity properly for this? and also how do I make sure that my visuals, (characters, tiles etc) would be considered optimal for this?
Or am I approaching things wrongly by asking this questions? How would you approach this situation?