The smaller the run, the larger the proportion of the floor space that's taken up with feeder, drinker, dustbath etc, plus the circulation space needed around each of these. You need to plan for an area where you can segregate new birds, or injured or sick birds, I.e. It's good to have a way of screening off part of the run temporarily when necessary, whilst still leaving enough space for the rest of them. A roof is really worthwhile, stops it all getting muddy and messy in the Welsh rainy winters. Chickens hate wind and rain, so protection is really good. My roof is clear corrugated plastic, (Rick has a beautiful tiled one) and most of us with covered runs use Aubiose (shredded hemp stems sold for horse bedding) on the floor and in the coop and nestboxes, as it's highly sbsorbent, doesn't go mouldy like woodchip, makes poo picking very easy and is warm, soft and pleasant for the birds. Chickens are often quite nasty to each other, and if space is insufficient you could get feather pecking, bullying, boredom and other problems. Nobody ever says 'I wish I'd made a smaller run!'
It's also good to give them some long perches made from 2"x2" timber with edges sanded down, as they love to sit companionably and look out on the world outside,
I'm going to move this thread to General Chickens, O.K? Please reply on that section after this.