I never heard of such formulae for animal forms (hit locations of course),
but I don't know that much RPG systems.
But with the right data (weight, volume, a characteristic length)
you can creat them on your own.
And then change the "flesh" to Eog.
I started looking for a scorpion, but couldn't find the needed data.
I once knew some biologists who work with cockroaches,
I believe they could have provided the needed data.