Relief Vent

Section 1 of the OBC:

Relief vent means a vent pipe that connects at the upper end to a vent stack and connects at the lower end to a horizontal branch between the first fixture connection and the soil stack or waste stack.

7.5.6.1.(2):

Every relief vent or circuit vent shall be at least 2" size.

Unfortunately this is all you can find in the Code about sizing relief vent.

Relief vent is installed between a soil stack and and the first fixture installed on a circuit vented branch

Relief vent is installed between a sanitary building drain and and the first fixture installed on a circuit vented branch