One big one would be ineffective, because all the mafia would be also be able to see the paranoia and react accordingly. (number of mafia + serial killers + 2) seems reasonable (so I think eight is the ideal size); last game it seemed the members of paranoia chats were disproportionately mafia, so it's better safe than sorry.
The unlimited freeze makes paranoia chats less likely to be effective in general though. Oh well. I don't think they're net negative for the town, but they aren't much help.