You can see a scientific calculation on kite sizes in different winds, correlated to what I use myself.
In the low end nothing is easy and very uncertain, as the apparent wind and type of kite and board etc means so much, but above this point you can take the curves as a given quite precisely, which size has the same power in more wind, just correlate it to your own "power" liking of course.
Push the line up or down according to your own preferences, meaning if you like to ride a TT lit and same weight, you take maybe two kitesizes bigger than I do on a waveboard - so it just means you should follow a line like if heavier.
Regarding temperature and height, see this post/graph from 2003: viewtopic.php?f=1&t=5159&start=10
And no, humidity plays no role regarding how powerful the wind is
Copied from an old 2005 post I made http://kiteforum.com/viewtopic.php?f=1&t=2343149
Regarding moist air and power (power/lift is directly proportional to air density/weight) :
Dry air, 0%, is HEAVIER than moist air, 99%
This is just the opposite of what everyone expects.
The reason is the difference in weight of the molecules.
"Moist" in the air is almost half as heavy as the air itself, so to speak.
The molecule weight of water is 18 (H2O = 2 x 1 + 16 = 18).
Air is 78% Nitrogen and 21% Oxygen and the last % some other gasses.
Both in pairs.
Molecule weight of N is 2 x 14 = 28, and of O is 2 x 16 = 32.
Because the number of molecules in air/gas is the same (it is, believe me) - the air without "moist" is heavier than moist air
99% moist air is only about 2% lighter than dry air though, so the difference can not be felt by us kite or windsurfers