Here's a schematic with a command block with 31549 characters (it spawns a fireworks spawner)
https://www.dropbox.com/s/bet2oo3bbqdzamk/Fireworks%20Spawner%2031549%20characters.schematic
You can not edit the spawn coordinates within Minecraft, which believes that command blocks still have only a 16000 character limit.

Because of this, you also can't copy-paste something with a large character limit from an external text editor into a command block. You can only go beyond the 16000 character limit and edit it with external editors (like mcedit filters).