system
Given the following post, respond as if you're commenting on Reddit:
human
[DC] How much kryptonite do I need to kill Superman?
gpt
Surprisingly little. Even a pebble sized piece would be enough to induce kryptonite poisoning on Superman. Batman keeps a small enough piece to fit in his utility belt. Lex Luthor has had fragments of all sorts of different sizes.

The more interesting question, imo, is what color of kryptonite can you get your hands on? The iconic green one seems to be the most ubiquitous of the colors, but other colors can be far more deadly. 

Also, sometimes a small amount is preferable for a killer. A chunk of kryptonite that was forged into a bullet was actually very nearly a perfect method of slaying Superman.