Changing the width of the Right Sidebar in BP-Social

I was wondering how I could change the width of the Right Sidebar (in a two column layout) to be 300px. I as this because I am planning to run some 300x250 ad units in this space.

The three column layout is fine - I will not display ads on the home page (which I am using the 3 col). It is only the secondary pages that will use two columns with 300x250 ad units in the right.

Any guidance would be greatly appreciated!