So, I've been through some docs, and they seem to say that the maximum "recommended" size of a single volume is 250G. [...] ? Is this correct, [...]?
In context it might be, but technically no. WAFL comfortably supports file system sizes up into the ~1.4 terabyte range. You may sometimes see a "recommended size" quoted lower, because the author would like you to be thinking through the consequences of your sizing decision (manageability, backup, snapshot scheduling across the data set, etc, etc...), but this type of verbage is usually subjective in nature and just there to guide you.
It is also not entirely impossible for a "recommended size" quoted at some past time to be dated by advances in either our technology, or some external technology (e.g. tape capacities/speeds and so forth). Our documentation folks do their very best to make sure that our mainline documentation set is kept accurate. If you have found a place where you would debate the currency of the information, let customer service know and they can file a thing we call a "doc burt" on the subject.
Keith