The size of the database depends on the number of tunes. You can work out the size by looking at the size of the files in /drive0/var - ie, database, playlists, tags. Some people may fill 120GB by using 320kbit and have a smaller database than a 60GB user with 128kbit tunes.
MK2as have a lot more headroom with their extra 4MB. Mk1s are severely strapped for space as is, only having 8MB.
There is no hard limit, the database just takes ram from the playback cache - bigger database, less playback cache. When the playback cache gets too small, things can get upset.
Hugo