Scaling laws are an important aspect of biology. When you take a particular object (such as a human being), and make it twice as tall (while keeping the proportions the same) its weight will not increase 2-fold. It will actually increase by a factor of $2^3$ (8-fold)! A 6-foot person weighting 160 pounds, if doubled in height, will therefore be 12 feet tall and weigh 1,280 pounds if you kept the proportions the same.
There is a problem with this. Although weight increases $2^3$-fold, the strength of the bones would only increase $2^2$-fold. It means that the strength-to-weight ratio of the bones is half that of a normal person. A giant with these proportions would stress their skeletons more easily and be at greater risk for injury if they fell down. In order to fix this problem, you would need to make the bones wider in proportion to their length so that the weight of the person produces less pressure on the bone.
A well-proportioned giant would therefore be a rather wide, burly-looking person with thick arms and legs.
Potentially your bones could generate more compact bone, and less marrow, and would compensate reasonably well. But, one's muscles, tendons, and etc would also have to compensate. The knees, ankles, and hips would also take a beating.
Would a person that big have to eat about 20,000 calories per day?
Possibly. Think about large theropod dinosaurs like Tyrannosaurus rex. Those creatures demonstrate that bipedalism is possible for very large animals.
In order to reach that size (5-7 tons), a lot might have to be changed about human physiology. A better cooling system might be needed (surface area does not increase as quickly as volume does, so the heat-generating tissue of a large mammal has less surface area to release that extra heat from). Might such giant humans require elephant-sized ears for temperature regulation? More sweat glands? Or perhaps a lower average body temperature? Larger or more efficient lungs would be needed too (the diffusion of oxygen and carbon dioxide gases is limited by the surface area of the alveoli in the lungs).
I calculated my basal metabolic rate to be around 1,730 calories a day, so if you were to scale my mass up to that of a T-rex, my metabolic rate would would increased by a similar amount (~112,000-156,000 calories per day). If you went for the "lower the body temperature" solution to the heating problem you'd get less calorie burn than that, though. In fact, warm-blooded creatures expend around 90% of their caloric intake just warming their own bodies up. A cold-blooded giant might not be so bad!
The lung volume should be ok due to the fractal space-filling nature of the lungs, unless your creature panted like a dog as part of its temperature regulation.
Humans vary in height from around 4 feet to about 7 feet with relatively "normal" physiology, although perhaps head size varies less than other parts of the body.
Wadlow's greatest recorded weight was 222.71 kg (35 st 1 lb) on his 21st birthday and he weighed 199 kg (31 st 5 lb) at the time of his death. His shoe size was 37AA (47 cm, 18½ in long) and his hands measured 32.4 cm (12¾ in) from the wrist to the tip of the middle finger. He wore a size 25 ring. His arm span was 2.88 m (9 ft 5¾ in) and his peak daily food consumption was 8000 calories.
The cause of death is very telling... Wadlow died at 1:30 a.m. on July 15, 1940, in a hotel in Manistee, Michigan, as a result of a septic blister on his right ankle caused by a brace
The current record holder, Sultan Kösen at 8'3" is often photographed with crutches. However, since there are many basketball players over 7', perhaps the leg problems are not always found with tall individuals.
Answer compiled from here