Which size?
Perhaps see "COMBUSTION AIR AND VENTILATION REQUIREMENTS" in here....
http://www.partsarena.co.uk/baxi/System/DATA/Dx/DS1/installation/2547/I23-2547/I...I'm NOT a gas fitter so wait for someone who is to reply.
Quote:b. The CF125 and CF150 may be installed in a purpose designed or modified compartment.
There should be two permanent air vents, one at low level and one at high level, both communicating either directly with outside air or indirectly with a room or internal space which is suitably ventilated (See a.). Each vent must have a minimum free area in accordance with the following table:

A vent 20 x 15 has an area of 250cm^2 less any grill. That appears to be a lot less than the 440 or 528cm^2 specified in the table for the high level vent.