W ~ 10ft, L ~ 16ft, H 8.5ft. I think I just placed my phone on a stool that was there. Not quite in the center, maybe more towards back of the room. I was clapping almost next to it, didn't go like all the way to the other wall to "maximize" verb or anything.
Small rooms can have real reverb if they're totally empty like yours. There are probably flutter echoes in there too, but each is masked by the others having different timings. My two-car garage is a little larger: