The mini any man works because it's 20-30 bucks...and who's going to buy a toy you need a microscope to see? Lol it's different when it's a 400ish(what a 1/4 scale Antman would cost) or 600ish 1/4 giant man(in true scale).
Just look at all the guff given out over the mos superman or Thor (which are actually right to the film just not the comic). Giant man just wouldn't work at 18 inches. If he's only 20 foot tall in the film he would still need to be nearly 5 foot tall, or like 3 times the size you mention. Now using a 1/4 Antman as a 1/6 giant man..if that's where you where going....probably still wouldn't work..as again it's off by a factor of like 3. A 1:6 giant man would be like 3-3.5 feet tall I think.
I mean hulk is 17 inches tall and he's 9 foot in 1/1. So it would need to be at least twice hulk, right? And hulk is essentially the size of a normal 1/4 scale human. You see why it doesn't really work right?