Sounds like the maxlenght for cvar.string. There is a reason for this, the game has a huge amount of cvars and each one holds an int, float and string. If the string buffer were bigger for each cvar, the game would consume much more memory.
Why do you need so many bans? You can use wildcars for the IPs, so you might save some space. If you're interested in a more advanced banning system take a look at Shrubbot (Part of Tremulous) and port if to SG