All the vents that I've seen in recent years are 14"x14". In the past, there were some cab-over campers that had 24"x24" "escape vents" in the cab-over section.
On your trailer, are you talking about the sky light over the shower? Usually there's a non-opening plastic bubble over the shower. It lets in light and raises the ceiling to allow for more headroom. If that's what you're talking about, I don't think there is a vent cover sized to cover the sky light. Your bathroom vent should be a 14"x14" size, probably with a small 6" fan. Any commercial vent cover will fit that size vent.
2015 F250 6.7l 4x4
2014 Cougar X Lite 27RKS