Ridgerunner had some great questions to consider. Another thing to take into consideration is the type of soil that is under the coop and run area - does it drain well when you have rain/snow? Or is it clay that puddles water (that you won't want your birds standing in). Is it on a slope where water will run under the coop?
We have 12 standard sized birds (Rhode Island Reds, Barred Rocks, Black Stars, etc.). Our coop is sort of a hybrid - it's a walk-in coop that is slightly raised, the floor level is maybe 18 inches off the ground, with a couple of steps for us to get into it. It's 7 x 12 feet, with a 2.5 x 3.5 foot storage area on one end that is accessed from the outside - we store buckets of extra food, shavings, etc. in there. This gives us a corresponding 2.5 x 3.5 foot area inside that we can fence off for isolating injured or younger birds (sick birds are taken care of in a tractor that we keep near our home's back door). It is tall enough to stand upright in comfortably - the side walls are 5 feet tall, and the center point of the ceiling is probably close to 8 feet high, which gives a nice steep pitch to shed rain and snow. We have a full-sized door on the south gable end, which makes cleaning it out pretty simple - our wheelbarrow can fit inside if we want to. We have an attached 15 x 25 foot run that is partially covered.
We originally built the coop about 8 inches off the ground, but realized that the ladies were actually digging to squeeze under there. So we raised it up enough to deter predators and pests, and so we can see what they're doing in there and access stray eggs or a sick/injured bird if need be. The posts are set on concrete piers (I think 8 or 9), so we could move the coop if we really needed to. Thanks to zoning and neighborhood covenants, if we wanted to put it on a permanent foundation, we'd have to get all kinds of permits - having it raised up makes it not a 'permanant structure.'
We get considerable snow build-up here (Utah), so we're banking the open sides with hay bales - they'll still be able access it from their run, but the hay will keep the wind and snow out. Should help keep the coop warmer, too, since the air underneath won't be as cold.
All that said, we love our coop! It's looks like a nice little shed. I like to walk inside every day and see how they're doing, check food and water, etc. It's easy to clean. It's still not entirely finished, but it's insulated and well ventilated and the chickens love it.