
Hi, welcome to the forum, glad you joined.
We are not trying to be mean or harsh, just trying to warn you. A lot of people do use the prefab coops but there are practically always issues. Most of them are not well designed for chickens, but are closer to a generic "pet" house that they added nests (usually too many) and a roost (often in the wrong place). If you an find one and give us a link we can critique it for you but we are not likely to be favorable. Occasionally you see one that isn't too bad. Since we don't know what country you are in it's hard to even suggest where you might get one, let alone what your weather might be so we could comment on what kind of coop.
If you follow the link below in my signature you can get some of my thoughts on how much space they need. Some of those things will not apply to you but many will. For example you may integrate new chickens later, that takes more room than if they are already integrated. Aart as some interesting links too.
With a maximum of eight chickens you can probably (depends a lot on climate) get by with one of those relatively small elevated coops. Anything over eight and I recommend a walk-in coop. You need to be able to reach everywhere inside the coop and that gets really challenging if you get much larger than a 4' x 8' elevated coop. If you are in a climate where they will be restricted to the coop only for extended periods of time a 4x8 may be too small for eight.
A shed is often a good option, whether it is a used one you find or one of those prefab ones you can put together. Don't forget you will need some type of foundation. To convert a shed to a coop you add nests (two nests for eight hens should be enough), roosts, and a pop door. You can use a human door for the chickens to go in or out but a small pop door keeps more of the weather out. You will probably want to build a run.
Regardless of where you are you need ventilation, even in winter. Where you are and what the coop looks like can influence how you go about that. Depending on your building skills this could be the most challenging part of the shed to coop conversion but there are ways. With those prefab coops you often need to add ventilation anyway.
Another option is to go to the "Articles" tab at the top of this page and look under the coops section. There are a lot of coop designs to chose from, often with instructions on how to build it. You could maybe build it yourself or possibly hire a handyman/contractor to build it for you.
Once again,

to the forum. People do this all the time so don't get too discouraged.