Let's not go too far here.
NOEDIT:
1MB = 1024kB = 1048576B.
The example ban
Nullable 23618 Jack Noir 29019 109.165.21.54 thanks -1 -1
has 58 characters (note that this depends on things like banner and banned name/blid, banned ip, ban message and ban length) and, since Blockland uses latin1/Windows-1252 one character = one byte.
This means that I could ban 18078 Jack Noir/Crystalwarrior about 18078 times while still having my banlist fit within 1MB. Note that this number is probably incorrect in your case, since A) the ban message is rather short, B) JN/C has a pretty long IP, C) morons tend to have longer names and D) non-permanent bans take more space. However, all in all, this is unlikely to matter in the long run.