If you're going to be stealing space from existing rooms, it may be practical to go for a long, narrow shape. My brother added an ensuite to his bedroom years ago and they took space from the adjoining bedroom, along one wall. It was about the minimum size you could possibly have, with shower at one end, toilet at the other and a narrow basin on the wall in the centre, opposite the sliding door entrance. I'd guess it was only 1m wide, and probably about 3.5m long.
It's really going to depend on the existing floorplan as to how big and what shape you can have.
There are lots of space-saving ideas for small bathrooms though. Look at things like shaving cabinets recessed into the wall cavity and vanities with semi-recessed basins. Think about where towel rails will go too - often that's not considered until the building work is complete and then there's nowhere to put one. And if you're not able to add a window, make sure you have a good skylight - dark bathrooms are awful.
There are a few tips here on minimum space required for comfort:
http://www.reece.com.au/create/WhatToCreate/UseOfSpace
This looks pretty good. The frameless shower screen, plus the floating toilet and vanity, give the illusion of more space:
Another small room, but they've managed to fit in a bath:
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If you type small bathroom layout into Google Images, you'll find lots of ideas.