As in "an echoey room". People do use this word in speech, but it isn't proper in writing. I thought of "echoing", but that implies that something is currently making an echo, whereas what I'm looking for would mean that if there was a noise in the room, it would echo. Are there any real words for this?
Reverberant - having a tendency to reverberate or be repeatedly reflected; "a reverberant room"; "the reverberant booms of cannon".
"echoic" would work in a narrow technical sense, but you could figuratively expand it, if you were so inclined. E.g., Apparent distance of sounds recorded in echoic and anechoic chambers.
Resonant, lively, resounding, reverberant all seem useful in various contexts.