Central heating door ventilation ?

We have had central heating installed with 2 zones,bedrooms and main living area. We have the zones as bedrooms being the common and the living area as zone one (we have fireplace in living area we use that's why zones are in reverse) the return in in our hallway roof. We like to close our hallway door at night to keep the cat out of the rooms, but were told we need to put a vent in the door if we want to having the living room zone running so air can reach the return. What I am wondering is how big I should build the vent? I have only come across small ones at the store say 5cm x 50cm. It does seem small, what would be the best approx size?