You have good timing, since at least one of those questions I spent a while chatting about with someone for a game recently. For extreme heights, you can scale any shape up proportionately with the square-cube law. However much bigger something is as a percentage than it was before, it's (that many times) cubed heavier.

new weight = old weight * [(new height / old height) ^ 3]

You can shuffle around the placement of the variables depending on what you want to calculate, but you can use a baseline reference of different weights at a set height to figure things out for other heights. EX

Start with 5' 6" (or 66 inches) for your baseline, and say 'normal' is, oh, 155 to 185 (just picking numbers from a BMI scale). You can plug that into the equation to see what 'normal' would be for a 20' (240 inches) tall individual with the same proportions-

Lower bound = 155 * [( 244 / 56)^3] = 155 * (4.357 ^3) = 155 * 82.719 = 12,821.449 lb

Upper bound = 185 * [(244 / 56)^3)] = 185 * (4.357 ^3) = 185 * 82.719 = 15,303.019 lb

So, using our baseline, we can scale set points to any size. A 20' tall individual with the same basic body shape as our baseline would be underweight at under 12,821 lb, overweight at greater than 15,303 lb, and normal weight at anywhere between those two markers. The weight of really big things is something that's often underestimated, but scaling up in three dimension increases volume and weight really fast!