I am trying to adapt the game to all the screens so I tried to change the window size (and reposicionate GUI elements) but how could I change the camera size?
Because if I write “yes” when I change the window size the area is cutted and the center of the camera is not the center o the screen, if aI write “no” then the game is stretched 
Basically what I want is adapt the game to all the resolutions (doing reescale but fit).
Start from a higher resolution. If the highest you want to work with is 1920x1080 (like me), start there and scale it down. It’ll take a lot of time to get several resolutions configured, but it’s possible. Use margins for variations in the aspect ratio if you plan to support more than one ratio like 16:9, 16:10, and 4:3. Personally, I’ll only bother with 16:9 and some 16:10 because those are the most common.
I haven’t gone in depth with implementing this myself, but this is my basic plan on my current project. I’m still figuring this one out myself. 
Maybe someone else will have some input to go into it more than I can, which I’m interested to know as well.