At the airflow rates I use 75 is fine double it up if you need to or put 2 vents in. I I have just done mine, I thought about 90, for one reason it seemed cheaper, but when you actually look the 90 stuff is less common and would have probably turned out more expensive to source everything. I think 75 is winning, so it is far more common and that must be because it is adequate, plus the fact it is easier to run, bends can be easier, narrow spaces under the bath etc.
To be honest I do not like the rigid or semi rigid ducting, it seems to convey the noise very easily as an eco chamber, even around bends, I would prefer it to be done in closed cell foam.
