After some research on the net, I could not find information on the ideal size should my photoshop document to my theme adapts to any width screens.
You didn't find it because there's not one ideal size, since the width of your theme will vary. It will depend how flexible your theme will be, if it will also fit mobile screens, etc. So you should decide how big or small screens your website will be presented in (ideally desktops and mobile devices) and make a layout that can adapt to the different sizes.