Best to do it when they are less than 1/4" around. It can be done when they are bigger, but they get a little woody. Makes it harder but it can be done.
Think of it like a solid piece of wire. You can bend it back and forth, back and forth and it will eventually break. That's what you want to do to crop.
Pinch it where you want to crop, squish the stem with your fingers a bit, the bigger the harder you have to squeeze, then bend it back and forth. When it creates that kink, that's what you're looking for. Then it'll knuckle up nice for you.