It is clear that an organism can only grow so large before the systems required to keep it alive need to become impossibly efficient. Circulatory systems, respiratory systems, nervous systems, etc. have to constantly fight against gravity, atmospheric conditions and resistance.
I'm interested in gravity more. I read an unsourced statement somewhere that on Earth, an animal cannot grow to more than the size of the Pentagon due to the gravity (weight). Are there any plausible models or rough scales representing the relationship between gravity and land animal size? Are they linear? Logarithmic? It don't require perfection, I just need something more concrete than the square-cube law.
(Assume that atmospheric pressure and composition remain fairly constant.)