Place them on a low-gravity world.
It's definitely possible to create anotomical structures on the size scale of a giant. Large therapods had legs of the right scale for such a beast and some, like therazinosaurus, even had arms approaching those that a proportional giant would have. However, it's probably not possible to create an anatomically correct giant of that size that lives on Earth. The biggest problem with having a 50 to 80 foot tall humanoid is blood circulation. Unfortunately, there probably isn't much you can do about blood circulation while remaining 'humanoid'.
Looking at the lower end of your range, around 50 feet, we encounter sized at which your giants are about as tall as a dinosaur like Spinosaurus is long. However, Spinosaurus and other gigantic carnosaurs had mostly horizontal bodies, with their heads about level with their torsos and hearts. Sauropod dinosaurs also follow this general body plan, though some probably raised their heads to some degree, at least 20 feet or so above their hearts.
The easiest way to get around the problem of blood pressure from being tall is to weaken gravity. On a planet with only .25 g, for example, pressure differentials for a 50-foot high giant would be similar to those faced by a 12-foot high humanoid, which is well within the realm of possibility.
Beyond that, you should expect your giants to have proportionately shorter limbs, since long limbs would be effectively more fragile for a huge creature, gigantic barrel chests to contain proportionately larger hearts (blood pressure is still a problem, even with our lower gravity, just not as much of one), and proportionately thicker arms and legs.