Okay, so what I remember… the movie had a very a low-budgget 1960s look and feel to it. The story was about a bunch of rebellious teenage characters taking some pills that turn them into 20 foot tall giants (“One pill makes you larger…”). They were in some kind of dilapidated theater when they grew, and so when their clothes (which remained normal size) tore off them, the girls bashfully draped themselves in the stage curtains.
After making some new clothes (or perhaps there were some enormous theater costumes lying around conveniently? I can’t remember), the group of teen giants then go stomping around town, terrorizing everyone and generally being bullies.
Finally, some “good” non-countercultural teenagers create a shrinking potion. Using a jeep, they drive around close enough to dose the giants in the potion. The giant teens begin shrinking back down to normal size. They then stood naked (their giant-sized clothes no longer fitting them) in the town square and facing the wrath of all the good townsfolk whom the teens “stepped on” when they were giants.
Rings a bell with anybody?