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: 
To add to Tim B's answer, rather than to compete with it:

**Eyes:**

Eyes do not generally scale with the rest of an animal. If you examine a mouse, it has eyes that are proportionally bigger than those of a cat, which are proportionally bigger than those of a human, which are proportionally bigger than those of a whale. Eyes have an optimum size, above which there is no significant advantage in a further increase in size, however when smaller than optimum, any increase in size is significant.
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.