With 2 teenagers, the time has come to consider an ensuite. Space is very limited. We want to try and squeeze it into our master bedroom (which is quite large). All we want is a shower, toilet and basin. Boundary restrictions do not make it possible for us to build on.
Are there any minimum dimension requirements (width and length).? Anyone have experiences to share?

Not sure about regulations but our children's ensuites are all 2m x 2m. They all have a toilet, a 900mm vanity and a 900mm corner shower unit which all works brilliantly. Still enough room to dry off in front of the shower etc.

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:

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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.

In the unit we are at the moment, there is a bathroom that is 1.8 x 1.7 m inside dimensions. It has a shower (900x900, with corner cut out), toilet and basin. This is fine for one person. You could probably go smaller if you had to. I think 1.6 x 1.6 could be made to work.

In the house we are currently building, each of the children's bedrooms have an ensuite. It took me weeks to juggle things to get the smallest size whilst still being comfortable - for one person. The inside dimensions are 1.324 x 2.204 m. The shower is only 760 mm wide, but is 1324 mm in the other dimensions (with no door, just a 750 mm long panel). I've mocked it up and it's OK even with elbows flying while washing your hair. The basin is 720 mm wide, which is OK.

These are about as small as you can get.

Our master ensuite is large, because the design lent itself to a large bathroom.

Of course, in Venice these would be considered large bathrooms. We stayed the top level of hotel there and the room was built into the roof. The bathroom would have been about 1.2 x 1.2 m. There was no shower screen; just a shower rose in the middle of the rom and a toilet on one side and a basin in the other. So when you showered water went on the basin and toilet - but they can handle it.

Fitting into an existing room as others have pointed out will probably work best with a long narrow ensuite design. I'm sure you could get away with 1.5x2.5 metres quite comfortably allowing for a 900x900 shower down one end, with a toilet at the other and a sink opposite a sliding door. Depending on the length of the wall that you put the ensuite on, you may be able to capitalise on the remainder of the length of the wall to put in a small walk-in robe (to utilise the full width) as there's no point in having a very long bathroom just for the sake of it (it could serve to make it feel more narrow and like a corridor).

Would be worth following up with casa2's design, as it sounds like a lot of thought went into designing the 1.324 x 2.204m ensuites.

If you need to go smaller than 1.324/1.5 m in width then I'm sure you'd get away with that also.