Make sure you get a brilliant tiler, never skimp on the quality of workmanship, it doesn't matter how much you spend on tiles, if the tiler is not a perfectionist it will look shit.
on as many walls as possible at least one long and one short, from about two thirds up, put a mirror, you will need to make templates so that the local glass supplies can cut and bevel them,very reasonable prices, where the mirrors meet will create a feeling of space,
Use light large oblong tiles,, for the remaining walls, best rectangular tile pattern for small space
Either rectangular contemporary engineered cork flooring (trust me it no longer looks like cork, and is the greenest product going) on the floor and as a bath panel, continue the floor patten, up the bath panel this leads the eye to enlarge the floor space.
Make sure the wall and floor tiles match closely in colour, then the eye won't jump, it's better to mix textures than colour in such a small space, use another finish to take the eye round.
down lighter in rows on the ceiling placed so the reflection evenly spaces , also help to widen the space,
keep fittings simple,
Remember to fit Aquapane behind any bath or in shower cubicles,
and make sure any light fittings, or other electrical are bathroom regs ip rated, a part p certificated electrician must be used for anything that is electrical in a bathroom, you must get the certificate and keep it for when you sell your house.
If you want to see good use of small space design search "ensuite luxury yacht", very high end expectations in small spaces.