there are also those 30" batman and superman figures that could be used as well. not much in the articulation department, but would make fore an imposing display piece, that wouldn't be as cumbersome as a whole friggin' mannequin...
Actually customs those size are usually but rarely done with mannequins. You can pick one up around $100. Not that far off from some figures. Decently articulated ones go up in price from there.
He can get up to 100 feet (wikipedia). The average 6 inch figure is suppose to be 6 feet. 100 inches would be the tallest you'd want to go which would be 8 feet tall. You could just dress up like Pym and act like you were Giantman.
You can also go with a 12" and extend the limbs or an 18" Spiderman. I've made one with the Spidey. Takes some sanding and sculpt to smooth but looks great and a used one usually won't cost as much as a BAF Giantman.