the highest person

You need 7-foot.

However, I"m not sure what you mean by "stacked." Are they each standing on the shoulders of the person below? Stacked sounds like they are perhaps lying on top of each other, like fire wood.

I'd also say "The person on top is waving his right hand." The person on top, not The highest person.

No, don't use "stacked" at all. That sounds like someone else placed them there. I suppose if they were toy soldiers, you can say that the child stacked them one atop the other, but people are not stacked by another person.

 Four 7-foot men formed a human tower, each standing on the shoulders of the one below, and the one at the top/on top, waved from 28 feet up in the air.

Grammar Geek

28 feet

CJ 

I usually wave with my hand at about the level of my face, I think. Although, I may extend my hand way up over my head in a case like that, so it could be from 30 feet if that fellow did the same.

By the way, I have an errant comma in my post. Can you find it?