The theme uses different measurements for column widths, most likely in order to make it responsive (i.e: so it works well on various screen sizes and devices).
The sidebar area is not set in pixel, but in em.
The main content area is also not in pixel, but in percentage.
This makes it rather hard to put a fixed pixel number to each column, but you can play around to change the default size. If you decrease the sidebar width, for example, then you add the main column width.
The original value for the sidebar is 17.4em, while for the main column is 66%. Here's an example CSS I did to tweak the widths: