Attack of the 50-Foot Whatever/Analysis
Being able to shift one's size, or even just being differently-sized by nature, has its benefits. However, it also has its drawbacks. For instance, the ability to shift one's size would require 1) an ability to change one's mass so that one does not blow away or sink into the earth (due to the shift in surface area and pressure), 2) because of the Square-Cube Law, one would need Super Strength to be able to support the increased weight, as well as the increase in pressure on the body (for example, if one were to go from 5 feet to 50 feet, the result would be a thousandfold, or 10^3, increase in mass and therefore weight, but only a 10^2 increase in surface area, thereby increasing the amount of pressure on one's body (or force per unit surface area) tenfold. Go big enough and you'd likely barely be able to stand, let alone fight.), 3) again because of the Square-Cube Law, one would need to vary their internal body temperature to avoid freezing to death at small sizes, or dying of heatstroke at large sizes (get ten times bigger and the increase in mass will cause a thousandfold increase in heat generation, but the increase in surface area will only increase the rate at which you dissipate heat by one hundredfold). Alternatively, the ability to survive a superhuman range of temperatures would also solve this problem, and 4) the ability to adjust the performance of your vital organs to accommodate your body would help as well. Having an innately different size would require these as well, at least to a greater extent, but would be "always on" instead of on the fly.