The bigger the better
If you are building your own, detached house, always make the bathroom as large as possible, even at the expense of other rooms. If possible, the best solution is to make a salon bathroom, which will be used for hygiene-related activities as well as serve as a great place for relaxation after a hard day.
A large bathroom will easily house two washbasins, a bathtub, a shower cubicle, a toilet bowl and a bidet. The perfect solution is to designate some space where you can rest after taking a bath. Such a concept obviously requires more space, so the decision about the size of your bathroom should be made already at the stage of design engineering, and should not be based on "wherever it fits" principle. Fortunately, bathroom in newly-designed houses is not a second-grade place anymore, it has gained the respect it deserves.
More bathrooms
A large bathroom with divided functional zones is simply flawless. But it is even better to have two bathrooms, so if you are planning to have a toilet apart from the "main bathroom", consider the idea of extending that toilet. Even in a small space you dedicated to such an extra toilet you can install, for instance, a shower cubicle, which gives you an additional bathroom to be used by guests, for example.
Open to the world
A bathroom can be designed with a window, but also with a door opening to the terrace – the shower cubicle will provide refreshment to people sitting outside on hot days.