Just asked DP who is a tiler and does heaps of bathrooms, he says 1 x 2.5 would be plenty for a usable ensuite. 1 x 900 for the shower at one end sliding door in middle (900) then toilet at the other end with a vanity opp the sliding door. The vanity would be 600 or so. that would be plenty functional and could even be slightly smaller. Gotta have a sliding door tho....
This exactly what we've got, but maybe the room is a little closer to metres long. Works fine, you don't have room for two people to be in there at the one time, but it's enough space for one to do what you need to do.