@kariboka @redstarfish I have prosody instead of ejabberd, but I believe it would be similar.
I have ~20 users most of which love cats :D , a strict limit on http uploads (5 MB and they get deleted after 30 days) and mod_mam / mod_muc_mam are configured with the default where messages are deleted after one week.
This was done mostly for privacy (I don't want to keep any personal data for longer than is required for people to download stuff on their multiple devices), but of course has the side effect of keeping the requirements light.
My prosody storage directory right now is 358MB, of which 335MB are in the http_upload directory.
edit: I'm specifying I'm using prosody because I'm using prosody's names for the configurations, but I believe that ejabberd has all of the equivalent features.