It’s all slightly relative but to my mind a ‘small’ coop would be one that is for less than 12 chickens. For many backyards a 3 chicken flock is a common number and to build a large shed for that many would probably be considered a bit overkill. Apart from the space involved, the main reason 3 is such a common number is that if all goes well it also should equate to around a dozen eggs a week which is about right for a 3-4 person family. If the outdoor space is limited then having the coop be movable is also a good idea as you can take your chickens to new cleaner areas of ground whilst also helping to make the grassy patch last longer (it will be mud soon enough if they are left on it for too much time).
Planning ahead is always a good thing and it may be that you don’t yet have any chickens and are now just preparing for getting chicks or point of lay pullets. If so there are a few things it might help to bear in mind before purchasing or building your coop with regards to how much space they might need and other general things like predators and how many eggs you might expect.