Combustion air vents will provide the required air to a 125,000 BTUH furnace in the basement. The vents provide free air directly through an opening in the attic. What size combustion air vents are required?

Combustion air vents will provide the required air to a 125,000 BTUH furnace in the basement. The vents provide free air directly through an opening in the attic. What size combustion air vents are required?