Space Inside the House
The coop plans inside chickencoopplan.com have a guideline in the description and in brackets in the title which specifies how many chickens can be housed in that particular coop. There is a lower and higher number given - in all cases those numbers have been calculated based on allowing 2.5 square feet to 4 square feet per chicken inside the coop. For example, if the coop plan is for a house 8x12 then the general guideline would be that it could house 24-38 chickens. The lower number (which allows 4 square feet per chicken) is obviously better for the chickens as it means they will have more room available to them.
The allowance of 2.5 square feet to 4 square feet is simply to provide a consistent benchmark across all the coop plans on chickencoopplan.com and is based purely on the general guideline for the minimum indoor house space needed based on the square footage available. It does not take into account the amount of ventilation (more is needed for more chickens) or the amount of outdoor space they will have available to them (if they don’t have a large amount of outdoor space then they will appreciate a more generous allowance inside).
The minimum recommended outdoor space is between 4-10 square feet each chicken (with the lower number really only being suitable as a minimum for bantam breeds). Those really are the minimum numbers - the more outdoor space you can allow for your chickens, the better.
If you live in a cold area where temperatures regularly get down to freezing and below you may want to consider adding additional insulation. Insulating your coop can be via temporary solutions such as adding more bedding, pinning up flattened cardboard boxes on the inside walls and piling straw bales around the coop during colder times. Another way would be to use thicker woods when building or to purchase actual roof insulation for example and then pack the coop walls with that and use sheets of plyboard or similar to seal it in. Even in the coldest weather though it is important to make sure there is plenty of ventilation.
If your coop is not going to be situated in a predator proof area then you will need to take extra precautions with the coop itself. For example, chicken wire is good for keeping chickens in, but pretty rubbish at keeping predators out (as it is easily bitten through or bent out of shape), so using a strong meshed wire would be preferential. Strong meshed wire carried on down 10 inches or so and then out a few inches (like an inverted 'L') should also help prevent anything from burrowing in underneath. Another way might be to use concrete as a base and/or concrete blocks buried below ground level all the way around their enclosure. An alternative to burrowing wire down is to make your coop a wire 'skirt' where some extra wire is laid out flat along the ground a few inches all the way around. In some cases you may also want to consider using a stronger or thicker wood (although if you are building a mobile coop that will make it a lot less mobile).
A 6 foot fence should in theory keep most predators out (and most chickens in) but it won't stop predators that can climb or predators (such as hawks and owls) that might want to swoop in - wire over the top of the run is probably the surest way of preventing that. If that is not possible there are distraction/deterrant methods which might help such as hanging reflective items such as cds around or use something like string / bunting over the run to deter from swooping in.
Other general chicken safety points are to always keep your chickens locked up at night, and to consider ‘free ranging’ them in your garden only when you can be out there with them - even if there doesn't seem to be any predators in your area you can be pretty sure they are there somewhere and it doesn’t take long for a neighbourhood dog to make its mark either. It is also wise to give your coop the once over at least once every year to check for anything that might need repairing / replacing.