Minecraft 1.7.5, Minecraft 14w11b, Minecraft 1.7.9, Minecraft 14w17a, Minecraft 14w18b
Here's a schematic with a command block with 31549 characters (it spawns a fireworks spawner)
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).