I'd describe it as
smaller than a small bank, employing no more than two or three people. I've never been inside one (not that I'm never hard up for cash) but I have the impression the client has access only to a counter across the front of the establishment, much as one has when visiting the dry cleaner.
My objection to the ATM analogy is that many ATM's are no bigger than a Coke machine, and are often situated singly, at outdoor locations. Your excerpt speaks of a "check-cashing
joint," which evokes a walk-in establishment, like a small bar, or restaurant. Ya'know, "The minute ya walked in the
joint - - I could tell you were a man of distinction - a real big spender!" - speaking of
strip joints, and con artists.