Krugerrand has most of it covered.
The most important factor is how you intend to keep the chickens; free range (out of the coop from dawn till dusk) ranging (sometime out of the coop and run), fully confined (no time out of the coop and run).
Get this wrong and the coop will never be satisfactory.
Getting this right requires knowledge of what predators one has in the area and perhaps more importantly, what one can do by law to protect your hens and what you are prepared to do.
For most people chickens are not like other pets and not like other livestock either; not many people want chickens in their houses should something outside go horribly wrong so getting the keeping circumstances right is very important.
Throw enough money at the project and one can cover most eventualities, but it will end up being a lot of money if one doesn't have the required skills.
You'll get lots of advice on this type or that type of coop. The probability is whatever type you choose will prove inadequate when reality meets romance as aart likes to state.
Lots of people decide often with almost zero experience to keep chickens. Most haven't thought further than getting eggs. Lots of these people find that chicken keeping isn't as simple as my pet makes me breakfast and think again about the the whoe chicken keeping business. One doesn't read much about when this happens.
It doesn't matter very much about all the recommendations on coop sizes, chickens will adapt to pretty much any form of shelter you provide. Give them a massive coop and you may well find them squashed in one corner, using a couple of feet of the twenty foot of roost bar you've thoughtfully provided.
Give them two or three nest boxes and they'll all want to lay their eggs in just one.
Start small and cheap. Buy a cheap prefab that will hold 3 or 4 chickens and find out what chicken keeping is like where you live. Don't go out and spend thousands on a massive coop because if you get it wrong you'll have wasted a lot of money.
In general BYC isn't a fan of prefabs. However, many people buy them and while it is true many people find the shortcomings unacceptable over time, during that time they will have learn't what exactly it is that they and the chickens want and make informed decisions for the future.
I've built over 20 coops (some of which can be seen on my coop page) for all sorts of fowl over the years and like many here on BYC would never have considered a prebuilt or prefab coop. Not one of these coops adhered to the square footage rule that gets trotted out here on BYC and the chickens thrived.
I have learn't a thing or two in the last couple of years and now I have a recycled plastic coop which I have altered to make more suitable to the keeping conditions I currently deal with.