I probably should have rephrased that to question whether all creatures based on large size will have a rehaul and rebranding as 'multi-tile' once they exceed a certain size. If we are doing math on the exact size of layers in comparison to existing animals that is.
If/when Toady introduces multi-tile creatures, I guess they will be reworked by hand. I already can imagine it will be horrible pain in ass, and probably enemies digging/destroying tiles would have to be introduced earlier, unless you want big monsters stucked harmlessly on smallest obstacle.
I mean, you probably could introduce some automation (if size is between x1 and x2, tile size 2x2x2, if...then tile size 3x3x3, if size is that and it is FLAT monster, then 2x2x1 etc), but many creatures would have distinct shape (giant snakes, anyone? 1x3x1) and behaviour (again, giant snakes will move and change direction differently than standard gigantic monster) that you would have to describe in raws. And don't get me started on neccessity of describing what body parts are on what tile...
I think multi-tile monsters (and any interactions of them with everything else in game - in world generation, fortress and adventure mode) alone will be one major release, so about 1 year. This is how much hassle they will be.