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I tried it, and the creature starts occupying 3-4 tiles, but it loses the ability to move (it behaves as if its speed is 0, even though it has enough speed) and looks terrible graphically. Also, there may be problems when trying to start a battle without the "Tactics" skill if there are only level 3 creatures in the army. In short, it's a bad idea.
Example 4*4 http://c2n.me/49JxUeq
How did you achieve this?