If a human is size 12, that's about 6' tall, for the average human male, right?

If an Igasho is size 18, that's 6 sizes larger than a human male at 6'. I'm guessing half a foot per size, .5 * 1, to get the average 6' human male, so the average male Igasho would be about 9' tall, based on that.

So a Faeling is size 4, which would be about 2' tall.

I just want to know if my calculations are correct based on the sizing and averages, or if I'm way off, and humans are 12' tall while Igasho are 18' and Faelings are 4'.

I would think that size also takes into account their mass, so the height differences would not be quite so large, but igashos would be thick, and faelings would be petite, fluttery things.

Your calculations are pretty much right. To make things easy, try using 6 inches per size stat. Also keep in mind, most women aren't 6 feet, so take that into account when you're dealing with other racial stats.

It's closer to a cubic relationship than linear. A 3' tall person is not half the size by weight, volume, or apparent size than somebody who is 6' tall, they are significantly smaller than half.

Well obviously, a Faeling wouldn't be as wide around as an Igasho.

I'm asking mainly because I'm tired of seeing 8' humans and 4'5" Faelings (written into people's descriptions.) I just wanted to know how people were supposed to interpret the sizes.

The faeling you see in Serenwilde, if I'm not mistaken, are two and a half or three feet tall - a little bigger than the two feet you'd get by straight calculation. Still, I'm sure there's a fair bit of room for variation, within reason.

Tae'dae are wider than they are tall. so a size 15 tae'dae would tend to be shorter than a size 15 Brood Viscanti.

I see size as more of a general size "feeling" rather than a height measurement. Like daganev said, tae'dae are wider. In the description of trills, they are quite tall, but have the same size as humans, so I assume they're pretty lanky.