The phrase "to fit the bathroom" means making the necessary changes or adjustments to ensure that something is the right size or shape for a bathroom. Full definition
Each comes with a fully fitted bathroom with hairdryer, individually adjustable air conditioning, as either a balcony or a terrace. (loveholidays.com)
Both bedrooms share a full fitted bathroom with a bath, shower, toilet, and basin. (travelground.com)
But you don't need to have a large en - suite or think about extending the space you do have, if you opt for fitted bathroom furniture. (yourhomeonlybetter.com)