Question: There have been a number of questions focusing on mythical creatures where the logical approach to answering has been to scale up an existing animal. For example [dragon's wings](https://worldbuilding.stackexchange.com/questions/313/how-could-dragons-be-explained-without-magic-which-characteristics-are-impossib) can be extrapolated from other flying animals or the [speed of an insectoid creature](https://worldbuilding.stackexchange.com/questions/20/insectoid-lifeforms-speed) from insects in our world.

However evolution has proved that animals do not simply scale up and down, mammals do not grow beyond a certain size unless they live exclusively in the oceans. Insects do not grow to several metres in length.

I believe that other ratios come into play such as power/weight and volume/skin surface area when it comes to whether creatures could continue to survive, move/fly and stay warm/cool at a larger scale.

Am I right? Is it an over simplification to say "A bird with a length of A has a wingspan of B therefore a dragon which is C long will need to have a wingspan of D"? What other factors come into play when scaling up real creates to simulate new ones?

Answer: 
**Insects several meters in Size**

Dragonflies were able to have a much bigger wingspan 300 million years ago (thanks to a much higher oxygen content in the atmosphere, plus warm as today in the equator), animals can actually be scaled quite a bit up.

In this case the limiting factors were feeding the organism enough oxygen to sustain itself, and provide it with enough heat to keep up its functions.

But the scaling up reached its limits here in the oxygenation of the tissue (though the free oxygen was multiple times higher than now), just as the growth of mobile organisms is limited by gravity and the ability of a pump (heart) to distribute enough oxygen in the biomatter for it to stay alive.

If a giant was to exist, it would die from the fact that its heart, rushing to pump blood to its head, would burst trying to stem against the pull of the planet. Only on a planet with weaker gravity could such gigantisms be observed.

<http://en.wikipedia.org/wiki/Meganeura>
I would suggest taking the appearances defining the creature and model it on a creature of the size you are looking for.  

For example if you were taking a husky and scaling it up to the size of a horse, take the horses proportions and then change what you need to, (tracing paper works well here) shortening the neck, changing the tail...and so on.  

Before everyone rants at me for not being scientific, this works as a basis and a broad overview.