Measure each outside wall's total length from one corner to the other. No space is absolutely square, so take 3 measurements of each wall; one from the floor, one mid way up, and one at the ceiling. The smallest measurement is the one to work with.
Make note of windows, doors, door clearances, toilet location, plumbing, electrical outlets.
It is critical to make note of these important locations as they will influence your bathroom layout and cabinet selections.