In terms of the cube square law gigantic creatures can only exist aboard spacecraft so giantesses would need a Brobdingnag or Land of Gigantics.
Surface tension would cause tiny creatures to drown in their own excrement unless they had a tubular organ for excretions and amount of food they would have to consume to stay warm would exceed production. .
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Hi! You're my new favorite giantess artist so far! I freaking love your work man! Keep it up!