Android 2D game multi-resolution

Hi! I'm making a simple mobile game and i'm aiming at this point to be available to Android devices.

I'm reaching the final stage of development and at the beggining of the development I used the size of background of 800x480. Now I started to think about multi-resolution (i know I should though about this in the beggining :( ). Maybe the 800x480 display was not the best of ideas... I searched a lot of foruns and unity answers but I did not find any tutorial of how to make a game to appear the same on different resolutions...

Can someone please point me in the right direction of how to do this? I have to change the sizes of my graphics. I want the game to run on all devices (phones and tablets). What should be the background size?

Thanks in advance!