There doesn't seem to be a set hight, as Giant Man (and others using his serum) has been seen to grow to the size of Skyscrapers (Ultimatum). However, one could assume that he couldn't grow infinitely, and he seems to stop at about 100-150 feet, at least when he was Giant Man.
In Ultimates 1, when he first uses the serum he pulls one over on Jan by saying that he grows too fast, however, that might be a valid problem if he was to grow too much/too tall. Also, if you take that Hawkeye is an average sized male, at least comic male, and put him at about 6' tall, he's at least a 10th, if not a 15th of Giant Man's size in Ultimatum (when he tries to climb up him) - so one could assume he was between 60 and about 100 feet tall then, and he doesn't seem to get much taller.
Hope this helps!