The default width of the site is currently 1200px. If you’d like to change the width of the whole site overall, then you can do a search for ‘1200px’ (without the quotes) and change it to whatever you’d like.
This will change the width of the main content area and the sidebar though.
If you want to increase the width of the main content area, but still keep the sidebar the same width, you’ll need to perform the above change to change the width of the whole site plus, as you pointed to in your original post, you’ll also need to change the columns in the template files.
The site currently uses a 12 column grid. It uses 4 columns for the sidebar and 8 columns for the main content area (4+8=12).
After changing the width of the site, you’ll then need to reduce the sidebar to 3 columns and then increase the main content area to 9 columns.
If you were to simply change the columns without changing the overall width of the site, then you’ll end up increasing the main content area and reducing the sidebar, which is not what you want.
I hope this helps.