An offset relief vent provides additional air circulation to a soil or waste stack with a horizontal offset. Determining where an offset relief vent is required and how to size it will depend on the code for your area. In Canada, we are required to install offset relief vents only if the offset is more than 1.5m long and the upper portion of the stack passes through three or more stories and has more than 100 fixture units on it. The connection of the offset relief vent is made at the lowest end of the upper stack after the last soil or waste connection, and the upper end of the lower stack higher than any drain connections.
Offset relief vents are only used in large commercial buildings, so the sizing should have been done by an engineer. Consult your local code for regulations on sizing offset relief vents in your area.