If I could design, I would make around 50s.f. with small toilet / bath (i.e. room 6'x 5' or 6' + toilet). This is enough for a bed and a wardrode.
Whether an air conditioner is needed depends on ventilation. I would leave the space for flexibility or future requirement.
I noted nowadays helpers look also for privacy and comfortability. Some do not want to share room (if they have choice). This is something we should take note of if we want to keep good helper.