Confessionals are just another component of the church building proper. Wood, drywall, cinder block? Whatever the church is made of.
They are not made to a singular model, except they are not terribly spacious, and the old private type have a sliding screen to obscure the identity of the penitent. The exterior is made to compliment the style of the church interior. There is either a chair or a kneeler on the penitent side, and chair for the priest on the other side.
With the advent of doing confession face to face, churches are building Reconciliation rooms, which are larger, and accommodate both people for either face to face, or private confession behind a standing screen. These can be up to about 10 x 10 feet square, but there are no hard and fast dimensions. It’s up to the architect of the church.