The CSS attributes relating to page layout/element widths begin at line 735 of the style-default.css file. Unless you are comfortable with CSS and willing to document your changes to avoid conflicts with future theme updates I would have to discourage you from making such changes.
Something to keep in mind(if you decide to modify the layout elements) is that when changing the size of something like the sidebar, you need to accommodate for that change in the DIV that goes with that corresponding sidebar.
For example – if you add 100px the element .col-3-4 you will need to subtract 100px from the element .col-1-4. The CSS file is commented very well which will be helpful, but you should have a decent understanding of CSS properties if you decide to take this on. One last thought – back-up/download the original style-default.css that way if something unexpected happens while you are making changes you can simply replace the modified file with a solid version.
Hope this helps.
