When deciding upon which size of exit sign you need there are some calculations you need to make. When your local fire officer comes to inspect the premises he is basically making sure that any occupant of the room or corridor will be able to determine the exit route in an emergency situation. You need to walk around the area or routes and decide where the emergency exit is and the maximum distance somebody could be from it. This will give you the maximum viewing distance. The maximum viewing distance deemed safe to be viewed from is given in BS 5266 Pt7/EN1838 and you will need to work to these guidelines.
Exit signs can be either internally illuminated or externally illuminated.
Internally illuminated signs have a light source, control gear and battery such as the ES8M or the LED MPL3M, they need their own electricity source.
Externally illuminated signs are generally a printed sign attached to the wall or hung from the ceiling that has a secondary light source illuminating it during an emergency situation such as a CS8M.
Depending which version of sign you use dictates the maximum viewing distance.
Internally Illuminated Exit Sign
175mm x 200 = 35000 mm (35 meters)
12 inches x 200 = 2400 inches (200 feet)
Externally Illuminated Exit Sign
175mm x 100 = 17500 mm (17.5 meters)
12 inches x 100 = 1200 inches (100 feet)
Greater viewing distances are achieved through internally illuminated exit signs. Externally illuminated signs are easy to install but they still need an emergency light to illuminate it during power loss. If you have limited space above an emergency exit door and can only use a smaller sign, but the room is bigger than the viewing distance, then you will need more than one sign to indicate the direction of travel.
This is for guidence only and whatever your specific situation it is always best to seek advice from a qualified electrical installer and your local fire officer to ensure that you comply with the local and national laws.