The 900mm refers to the edge of the tray i.e. the floor space you need to fit it. In my experience they tend to come slightly smaller so a 900x900 tray won't quite measure the full 900. obviously check this on the specific tray you order as they vary.
HTH
Hello,
I'm looking to fit a bathroom extractor above the shower and want to avoid going through a wall with the exhaust.
Whats the view on an fitting a vented roof tile, my bathroom guy wasn't keen and suggested the extractor might not have the legs for it?
I want to be as sympathetic...