Can we go from this:
Suppose over millennia (or as long as it takes) humans on Earth are artificially selected by aliens to be big and healthy.
Is there any theoretical reason that they couldn't grow to be as tall as large dinosaurs?
If the proportions (thickness of bones etc.) had to be changed, what would those changes have to be?
EDIT for clarity (I hope)
*These are intended to be giant humans. If someone found their skeleton, they would be able to identify each individual bone that occurs in a human skeleton. Imagine an archaeologist saying, "Wow! These are humans!". They should walk in an upright position rather than being able to use their knuckles like a typical ape. Relative bone lengths should be consistent within normal human variation but bone thickness or size of joint can vary in order to carry the required weight. *
A target height would be about 26 ft (8 m) but the taller the better.
They must be able to support their own weight and walk easily and bipedally.
The giant-sized humans should still be recognisably human even if their proportions are somewhat different - for example they might be stockier or spindlier.
The aliens will provide the humans with all the food and comfortable living conditions necessary. Only the healthiest and largest humans are allowed to breed. There is no manipulation of DNA, just selective breeding for size.
This occurs on Earth and on land so everything happens in 1G.
The tallest man to ever live was Robert Wadlow at 8' 11.1" (2.72 m)