I'm sure there's plenty of confusion about the why's and wherefore's of house orientation. The reality, as I have recently discovered, is that if want to live in an area that is older, developed, close to facilities, you take whatever orientation the block actually happens to be and do the best you can with it. Unless you're out in the country, or buying something in a new estate, you may not get a choice of orientation: we certainly didn't. And yes, our garage does have the best aspect - because there was nowhere else to put it - it had to face the street - but we live walking distance to shops (think of the petrol saved on not having to get into the car), we've got clerestory windows on the west side so no afternoon sun issues, and we're assembling a collection of sails for the south side, where the deck is, to provide shade from the late afternoon sun. Oh, and skylights for the kitchen. No, not ideal. But the location is, and the designers have worked around the constraints of orientation. And that's reality for a lot of us.

What do you want in the ensuite? Double vanity? Double shower? Bath? If so, what kind of bath? A separate / private toilet? I think the answers to those questions will direct you to what shape room will best meet your requirements. But, in general, a long, narrow design seems to be the most appropriate, with the shower at one end, sink/s in the middle and toilet at the other end.

i would probably try for at least 90cm but depends on the size and shape of the room, location of doors and windows and size and shape of other fixtures..can you post a plan of the room?

if you open the door in the middle back against the basin at one end and toilet at the other the minimum space to to consider would be about 90cm x 190cm and use the smallest toilet and basin depths but would be better to post the whole floor plan to see all the options