I have a 1997 Corolla with 150K miles on it. It burns oil a bit and I bought some anti-engine leak stuff. Since the car only uses 3.6 quarts for an oil change, should I use only half of that additive instead of a full quart?

depends...just how bad is your oil leaking???

I usually change the oil every 3-5K miles and it burns off around 2 quarts.

its sounds if you got a leak that is a good candidate for a replacement of all the gaskets and seals in the engine...not really hard work but time consuming...and one think about stop leaks...dont use them...rarely work and they can also mess things up more than it was before...