The layout I want will be something like this:

I want the area labled 'B' (Which will have a coloured BG) to grow automatically to the same height as 'A', which whil change in height from page to page. The width of 'B' will remain static and 'A' will dynamically adjust based on window size.
A is the main content area, B will be used for a new feed and C is where the page footer will be.
Is anyone able to give me some advise as to how my CSS should look to achieve this?