On Mon, 15 Nov 2004 03:57:09 +0000 (UTC), "Number 6"
 wrote:

>Size isn't everything so we hear. The dinosaurs were pretty big and now we
>have discovered small human creatures. So given the size of a planet there
>must be a physical maximum size for its mammals. For instance, for a given
>force due to gravity the heart would need to be big enough to circulate the
>blood.Same for trees - how high can they get? The fluids need to be pumped
>up to the leaves.
>Therefore how small can we get and how big can we get and if the planet had
>less could the creatures be bigger? (conversely for a large planet).
>

The subject you ask about is generally called "Scaling in Biology".   

There are a number of sources that you should read.

"On Being the Right Size", JBS Haldane, 1928)
  http://irl.cs.ucla.edu/papers/right-size.html

"Scaling : Why Is Animal Size so Important?", 
K Schmidt-Nielsen, 1984

and, of course,
 "Dinosaurs, dragons, and dwarfs: The evolution of maximal body size"
GP. Burness,, J Diamond,and T Flannery, 2001
Proc Natl Acad Sci U S A. 2001 December 4; 98(25): 14518-14523
http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pubmed&pubmedid=11724953
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