Foot between heights: assuming identical diets, absolutely. Different people metabolize and absorb things differently. Assuming ideal diets for everyone, I'd bet a foot would be near the limits.
Muscle mass, depends significantly on how much exercise. Some respond to it differently than others, so some would be more muscular with light exercise while others would be more with heavier. A difference though, yes; if somewhere near the middle, accounting for height / overall body build, a moderate amount of difference, but not a whole lot.
Intelligence, barring physical defects, I don't know. People certainly seem to have specialties, especially if you consider some of the "greats" of history as nigh-savants, but measuring an overall intelligence of a person is a nightmarishly subjective task. Very broad statement: not much of a difference. A lot of specialists are severely lacking in communication skills, a lot of artists in engineering, etc, and those deficiencies would have to be factored in.
At the end of the day, I'd bet we're all pretty darn similar. But I do lean significantly towards the nurture-over-nature side of the argument; do nothing, and you become a blob, regardless of your biology. It happens to animals too. There are, of course, some differences, but you have an incredible amount of control over what you make of you.