If a giant suddenly appeared in downtown Vancouver, it is likely their legs would instantly break from their weight as their physiology doesn't support their size. Once you get about 8 foot tall for humans, our bodies shape just doesn't work well. Most "giants" of legend were well under 7 foot tall. Giant compared to most people who likely only averaged around 5 1/2 feet tall because of relatively poor nutrition, but not anything near the 15-20 foot height of most giants in D&D. Fiends of all types would, at best, become humans that look a little odd and so on. Which is the issue I have from a fiction and gameplay point of view, I don't want an anti-magic zone to be able to incapacitate or immediately kill the majority of higher level monsters. Heck, a beholder would wreak havoc just by looking at things.
