A master bath, not a massive bath
Instead of planning a large bathroom with every imaginable convenience, you’re better off saving space for the sleeping area, a closet/dressing area, or even a small office alcove if you’re willing to bring that part of your life into the bedroom. Not only is it space you’re more likely to use and appreciate daily, but you’ll also have more money for the fixtures, fittings, and tile that you touch several times a day. You can fit an efficient, comfortable bathroom in a footprint as small as 5 ft. by 8 ft. if a sink, toilet, and shower meet your needs. A double vanity and tub/shower combo stretch the minimum-space requirements to about 8 ft. by 9 ft.
Code requirements dictate the minimum amount of space required for each element in the bathroom, but you’ll find that adding a few extra inches of elbow room or legroom around the fixtures makes the room seem disproportionately larger than the small increase in its footprint. For example, code requires a clear 21 in. in front of sinks and toilets. But 24 in. is an improvement, and 30 in. creates a very comfortable space. As you lay out possible fixture locations, remember that the clear spaces around fixtures can overlap; it’s not as if you’ll be using the toilet and the sink at the same time. Extending the maneuvering room around a fixture much beyond 36 in. is a waste of space because you’ll start to feel unmoored from the surrounding elements.