Just how big of a room does one need for XLFs?
In general, I think room dimensions are more important than room volume. If the room is fairly flat but small, the XLFs could sound quite a bit better than if they are in a large space with big bumps and dips.
I believe the minimum listening distance (in theory of course) is about 8 feet or so...but picking a single statistic is a dangerous thing. That said, I have heard big Xs which were 'honed' by the Distributor, and they sounded great in smaller rooms with short-ish listening distances of about 8 feet. It is one of the reasons I like them...the Distributor is great and gives me confidence to have them because in addition to repairs, general service, etc...I know how important set up is with ANY speaker...and having Pedro is like having 2 upgrades to a speaker setup by 'mere mortals'.