Insight said:[Hijacking]
Any damage that you have no way to resist (falling damage, etc) should be a Fort Save or die mechanic, with the DC based on how far you fall. Using HP to represent 'falling damage' is just plain silly.
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[Hijack again]
Instead of that, why not just change the way falling damage is handled?
What I'd be tempted to do is to scale both damage die and max number of dice (terminal velocity) by Size.
If we assume a baseline of Medium using d6's with a max of 20d6, Small would then be 15d4 and Large would be 25d8 and so on.
[/Hijack again]