In the bathroom, wainscoting height will vary according to personal taste and the layout of the room. Traditionally, wainscoting extends 45-inches from the ground including the top rail. Prefabricated wainscoting panels are 48-inches tall, so some homeowners may choose to go that high. Bathroom wainscoting may extend as little as 36-inches from the floor.
Wainscoting height in the bathroom may depend on a number of variables. Sink height, fixture position, or other design features might influence your decision. Matching the height of wainscoting in other rooms may be important. By no means are you expected to stick to 45-inches if that height doesn't look right to your eye. Adjust it until it suits your eye. It should be no higher than 48-inches and no lower than 36-inches.