Chicken Coop and Pen Size
If you do not have cover in the outside run then the more space available in the coop then so much the better bearing in mind they will be spending time in the hen house sheltering from the rain and wind. Guidelines are a minimum of 1 foot square for accommodation per bird and 1 metre square for run space.
The hen house itself is mainly for sleeping and laying eggs so extra space is not such an issue. However a coop of the right size for the number of birds will help to keep them warm in cold winters as the chickens will huddle together.
Perches should be a minimum of 25 cm (10 inch) for large fowl and 18 cm (8 inch) for bantams. Allow 30 cm (12 inch) between perches if more than one.
The bigger the outside pen the better. The run area can get very muddy and dirty with droppings in a surprisingly short space of time. The ground could get a build up of parasites over time too. In small runs the hens would soon get bored and could develop bad habits like feather pecking. Woodchip, straw and autumn leaves are great ground cover in chicken runs.
Height in the run is not too much of an issue, hens stay pretty much on the ground, it's getting in to clean it or getting to food and water that you need to think about. It is best to have two run areas that can be used in rotation to give the ground a rest and a chance for the grass to grow back.
Ideally, the hen house should be raised off the ground to avoid vermin nesting underneath. If it can be raised by about 18" or more, the feeder and/or drinker can be placed underneath. This increases run space and also provides a covered area for shelter in bad weather and a shaded area during the heat of summer. Without shade chickens can over heat and die.
Do NOT over stock, it is just as nice seeing 3 hens scratching about as it is to see 6!
Alternative coops can be made with Cheap Plastic Storage Sheds and Compost Bins.