Thanks for your comments Izzy, the reasoning behind a larger shed is that as the birds are going in earlier and being let out later, there is very little room for the birds to move around or stretch their wings should they want to, also there is also no way to provide lighting. Also if I am late getting over to put them in they tend to congregate in one coop (approx. 4'*4') which with 15 birds is a heck of a cramped coop.
My coops and run are situated on an allotment plot close to my home, this means that due to the dark mornings the birds are not let out until after 9am after my OH has taken our daughter to school. We cannot let them out before this due a problem fox that has taken several birds for neighboring plots recently.