This is in rooms with a 8' ceiling and 4' -6' tunnel. Rooms with higher ceilings or cathedral ceilings the light spreads out farther. You will need to put in a larger sun tunnel or multiple units.

See the cathedral ceiling in the living room below. 2 - 10" sun tubes would have lit up that living room if it would not have been a cathedral ceiling. Upgrading to 2 - 14" light tubes did the job of filling the room with natural light.

These are the main things to consider when choosing the size sun tunnel - skylight for your room.