The Max width of the body from a website

I need help!

I don't know how to setup a width for my website in the graphical editor... I want the PC view to have a higher width and the website to be completely covered.

Approximately like this website:

PS: Sorry for my bad english! :D

Thanks for all your answers!

Can't change preview width within the app but if you set your containers to be fluid in rh menu when selected it your browser preview will be full width