8x8 is a great size for that many hens (or fewer) because the plywood is already in 4x8 dimensions and you can buy 2x4's in 8 foot lengths...this means less cutting with a skill saw!! Less cutting is a good thing! Unless you have your very own professional carpenter building it for you. For your roof you can use 10 or 12 foot 2x4's so you will have some overhang for protection from rain...and...no cutting! Here is mine before the run was attached:
Here it is with the pvc hoop run attached:
I have a tarp over the top all the time for protection from the rain and in winter I also put plastic sheeting on the sides to block the cold winter winds and snow.

Here it is with the pvc hoop run attached:

I have a tarp over the top all the time for protection from the rain and in winter I also put plastic sheeting on the sides to block the cold winter winds and snow.