"The Amazing Colossal Man." Saw this long ago. I think it's exposure to radiation that sends this guy's pituitary gland into deep overdrive, as he grows to about 100 feet high. Anyone remember that?
The already mentioned "Invasion of the Body Snatchers" and "Invaders From Mars." Pure classics.
"Attack of the 50-Foot Woman." Pure 50's fun.
Not a particularly entertaining one, but who remembers the one about a severed hand crawling around wreaking havoc?
After school, I'd get home in time to see "Chillers From Science Fiction," a bay area TV station's afternoon show featuring mostly 50's sci-fi and horror flicks. Many kids of my age wouldn't have been exposed to these films as they were long out of the theatres, had it not been for the afternoon TV showings.