I had the same problem with adjusting the width on the mantra theme that i am using. I figured it out.
You can use the Mantra Setting, select LAYOUT and the Content/Sidebar width is set the "absolute" by default. You can change this to "relative". Relative means as you change computers the width adjust to that computers screen resolution.
Also, you will see a slider with a percentage. Change the percentage to 60%, and this will increase the width of the page, it allows for sidebar to be 20% of the width of the page. Then when you go to a different computer the width will automatically adjust to the new computer resolution screen and you don't have to change anything.
Suggest working on a smaller screen. As i am working on a 36" TV screen so although my website looked great on my screen, when I checked it on my laptop it was too big and I couldn't see my background. All of that is changed now with the above adjustment.
Unfortunately, my site is not up yet as I am still in the development stage.