I am remodeling an upstairs bathroom which has never had an air conditioning duct. The easiest way in my mind would be to tap off the current duct in the adjacent bedroom and install a new ceiling vent (the heat is baseboard hot water). My concern is that the current bedroom duct seems to be sized correctly, and if I tee into this line for a branch to go to the bathroom, then the bedroom may now not be sufficient. Should I increase the bedroom duct size while installing the bathroom branch? How big should the bathroom branch duct be? The bathroom is approximately 6'x9'. Thank you.

The bathroom is not very large. Tap off the duct and try it because increasing the size of the main trunk line with all the associated parts will cost big bucks.

Thank you for your thoughts. One clarification. The duct I would be tapping off is itself a branch with round flex duct, if this changes your thoughts. Also, what size duct would you recommend for this bathroom? Thanks again.