Im a new puppy owner--got the smallest size because mines a little Pom.  The pen is large enough it holds two full size rewashable potty pads, the dog turf I lay over one of them, her crate wbed, food dishes, toys  plenty of room for her to tear up all of the above.  The pen comes pre-assembled.  You merely open it up and put it where you want.  Can fold it back up and move it outdoors to a patch of grass for contained outdoor play.  My puppy loves having her own space. Its letting my cats safely get used to her too, and visa versa.