In my local area there is one signal box (Horton Road, Gloucester) controlling the convergence of two minor routes (Gloucester – Swindon and Gloucester – Cardiff) with the main Birmingham – Bristol route.
The box controls about 40 miles of track towards Birmingham, and about 20 miles in the other three directions.
There are one or two small boxes controlling busy road crossings but they are slowly being phased out.
Regards,
Chris