While my roof was being replaced, I requested that they install two roof vents. One for laundry and one for bathroom fan. They installed two of these. I just tried connecting the bathroom fan to this vent but, when on, the fan gets noisy and I can't feel much air coming through the roof vent. I'm concerned it's not compatible with this use.
The roof vent is about 4" in diameter at the base. Under the (non-removable) metal cap it appears to have a rubber flap that opens under pressure, covering a ~2" diameter hole.
One other thing to mention is that this is a shallow pitch roof (about 5 degree?) on a row house.