Have you looked on Pinterest? It’s my go to for house inspiration!
Also, I’d agree with the mirrors but a cheaper way to do it might be to use a piece of mirrored glass straight on to the wall, then tile around it - you see it quite often in hotels And it works really well, we did it in our last house.
Once you start looking at actual pieces to go in there (like the toilet and shower), get some masking tape and mark it up (inside or outside), so you can see how much room you’ll physically have between things (works well for me as I find it difficult to visualise separate pieces just on paper).
Wall hanging things (like the toilet and sink) so you can see the floor underneath will make the room feel bigger. Also, you can get very slim line hidden cisterns for toilets - again, because you can tile the hidden bit and it blends in, it often makes it feel much bigger.
If you are able to then underfloor heating could save you the space that a radiator takes up. If not, then you can get some small, slimline heated towels rails which would probably be fine for heating such a small space, and may take up less room than a tad.