Then you will need to install the child theme for Circumference and activate it. Using a child theme allows you to make modifications to theme files and being able to update the theme without losing your modifications. When you activate the child theme, you will have to redo your theme option settings because WordPress will see this as a new theme activation.
When you done this, you will need to upload a copy of this file ( header.php ) to your child theme and put it in the same place as the parent theme (Circumference) which is where the style.css is seen.
Look for this class: col-md-5 and change it to col-md-6 (or if you need it bigger still, col-md-7)
Then look for this class: col-md-7 and change this to col-md-6
The first one col-md-5 is your logo container and the col-md-7 is your menu container.
This theme is built on a 12 column grid (12 columns wide). So if you add width to one, you have to take away from the other if it sits beside it. Both containers side-by-side need to total 12. So for the default setup:
col-md-5 + col-md-7 = 12
So if you want the logo to be col-md-6, then it would be like this: