Correct resolution for assets

Hello fellows, I need to know what is the correct resolution for my character, NPCs, enemies and ohter things.
I’ve created a 1920x1080p image and drew some assets, but when I use it in my project, I have to resize tham to be part of the game world, but they are serrated and ugly.

I’m using .PSD format, my monitor resolution is 1600x900. Thank you guys and sorry for my english.

I’m assuming you’re talking about 2D sprites, in which case I suggest you read Pixel Perfect 2D | Unity Blog

For information about designing UIs for different screen resolutions, you should read Redirect to... title of new-page