The minimum vent size is based on the number of drainage fixture units served by the vent. For a secondary vent serving just the fixtures you listed, 2" should be sufficient. However, your code probably requires you to increase the vent terminal (from 12" or so below the roof line, on up to the vent outlet) to 3" or 4" to reduce the possibility of ice blockage (especially up there in the great white north). So it might be easier just to run the 3" all the way up. Check with your local inspector to verify vent and vent terminal sizing. Good luck.