What are the dimensions of the toilet room?

I don't think there is a code for this especially since there are not outside walls. If you keep the toilet room door closed when it is not in use, it can get noticeably hotter or colder than the rest of the bathroom. The extra vent makes it more comfortable. You can always close the vent if there is too much air flow. I would not worry about it.

Awesome, thanks Mike. Yes I am not too worried about it, I will likely just close it off. Is the consensus that if I close off this one vent it won't add any extra stress to the system and cause any premature failure? I have read this before, but to me it seems like it won't make a difference.

Closing one vent is not going to have much of an impact to the HVAC system.

I would love to have and A/C supply vent in my castle while on my throne!!! :-)

Not having an HVA/C vent in my toilet closet made a hugh impact. My master bathroom suite also has a whirlpool tub and a shower to go with 10 foot ceilings. After about three years, I notice mildew on the ceiling of my toilet closet. The moist air from the tub and shower did not get circulated enough and mold/mildew became evenly visible on the 3’ x 6’ ceiling. So, I added a timer to the exhaust vent to run 15 minutes every six hours which has solved the problem.