Is there a recommended size for vents in boiler cupboard doors?
Page 16 of the boiler installation guide PDF (Section 7.3 - Ventilation of Compartments) points out that no air vents are required in cupboards or compartments with your boiler.

This is because although the boiler requires oxygen in order to burn the gas correctly, your boiler has a double walled (balanced) flue and takes in all the air it needs through the outer part of the flue, drawing the oxygen it needs from outside the building.

See this OFTEC page for more information on balanced flues and cupboard air venting.