Thank you for taking a look at my problem; I accept that often what appears to be simple is not simple to implement. I however am not sure if I have described my requirement properly, so I have chosen to make a mockup image to better illustrate what I am trying to achieve.
Currently when I put content into my "Shop" page, the content appears left justified with a large space on the right as shown here(indicated by the red outline):
What I am trying to achieve is to have the content take up the remaining space on the right as shown here:
Is this unobtainable as you describe above?