The industry standard for RV roof vents is 14“x14”.1) OEM vents are often substandard, lacking cowling around the fan that would move more CFM.
OEM vents are very simple and will let rain in if left open. There are several ways to deal with this issue:
Be diligent about closing the vents manually. Easier said than done.
Install a vent cover; this will allow for ventilation during bad weather. It also has a bonus function of decreasing sun and weather damage to the actual vent. Be sure your chosen vent is has enough room for complete movement of the vent's lid. Application notes:
Select a vent with a rain-sensing function that will close the vent automatically, like the Fantastic Vent 807351. The manufacturer explains: “When dome (lid) is open and moisture contacts the sensor, the dome closes and turns fan blade motor off if it is running. When the rain sensor dries, dome reopens and fan blade motor runs if fan was running when dome closed.”4)
More expensive fans have features that some will find worthwhile:
Since hot air rises, you may get best results in summer by using the fan to blow air out of the camper. If you have a ventilation intake under your vehicle you may be able to pull in relatively cool air.
In winter you may want to run the vent on the lowest setting pulling air into the camper. This will preserve your warm air at the roof, contribute to circulation of air, and help control humidity.