There was this one thing I tried, once, for determining naga length.

I tried, first, to assume the female half's proportions were 36-24-36 (which is unrealistic, but hey, it's a good enough assumption to begin with). Then I looked for a snake whose girth was the same as that of the hips: 36 inches around.

Now, if you've drawn much, you'll know that a woman's center of gravity lies at just around the cutoff point. So, assuming said woman measures five foot six (which is unrealistic for proportions like 36-24-36, but hey, it's Felarya), you have 2 foot 9 in. of woman, and the rest is snake.

Using some old data I had, I picked up data about a green anaconda, of which the fattest ever was 550 pounds and measured 37.5 inches in girth. So we use half a python, 275 pounds, and add half a woman, 63 pounds... we'll assume now that the linear density of the woman is the same as that of the python (no offense to the ladies). So, if 63 pounds of woman measure 2 ft. 9 in., 275 pounds of woman measure 12 foot.

Total length: 14 foot 9 in. Give or take 4.36 times the length of the body from the crotch up, for the length of the body, and 5.36 for the whole body, which would mean... a naga's length is 2.68 times her intended standing height.

If you want to use the whole python (Mmm, naughty), you have 23 foot 2 in. of python per 2 ft. 8 in. of woman, a total of 25 foot 11 in to represent a 5 foot 6 woman. Her length would be 4.71 times her intended standing height, and 9.42 times her crotch to top of the head measurement.

Green anaconda data: http://animals.nationalgeographic.com/animals/reptiles/green-anaconda/

Actually, when I try to model the anaconda as a cylinder weighing 227 kg and measuring 9 metres, 25.22 kg/m of anaconda and with roughly the density of water (like any living being capable of swimming), I get that its cross-section, in order to approximate a circle, must have a perimeter of:

25.22 kg = 100 cm * X cm^2 * 1 g/cm^2

X = 25.22*1000 g /100 g = 252.2 cm^2

252.2 cm^2 = pi*r^2

r=sqrt(252.2/(3.14))=6.33

p=2*pi*r=6.33 cm*6.28= 39.7524 cm

1 in. = 2.7524 cm

39.7524 cm = 15.65 in.

That's a bit under 16 inches. I'm hoping that by using half a snake, you get the thickest section of the snake, the one that's as wide as the aforementioned hips... should there be such a section. Otherwise, you'll have to upscale the snake, which means, multiply the fraction of snake you want to use (1/2 or 1/1) by the desired ratio between hips and girth so they match.

In the aforementioned case, let's assume that we use half a snake, that the thickness where it's thickest is 24 inches, and that we want it to match 36 in. hips. Multiply the length of the snake (30 ft.) by 1/2, for 15 ft., and multiply that value by 36/24, for 22 ft. 6 in. You get 22 foot six inches of length to add to the torso.

In short...

Method 1- Multiply standing height by 2.68 (conservative estimate)

Method 2- Multiply standing height by 4.71 (generous estimate)

Method 3- Multiply either earlier value by desired hips/girth ratio (You need snake girth data to do this, and an intended hip measurement; gives relatively more believable results)