For a room less than 25sqm is not acceptable.
I am usually happy with something around 50sqm.
But lay out is more important than size only.

Depending on the occasion I prefer to have a suite with two separate rooms (as Pierre usually wakes up later than me). For a suite less than 70sqm is probably too small.

For city hotels one of the best lay-out I have in mind is Deluxe Suite in Bvlgari London (no powder room so no lost space but double access to the unique one from bathroom side and foyer side + narrow but long bathroom with lots of natural light and superb bathtub).