If you start night crawlers I would do pieces slightly larger than the space in between Mojos eyes.
Use a good pair of scissors to quickly snip the worm. Also if you snip the pice form the end of the worm
it will heal and not die. When you pick one up and rinse it off and observe it you will see a portion of it that is not quite half way
that is a bit thicker and a different texture from the rest of the worm. This area has the worms hearts. The area that is shorter after the heart is the head. Do not cut from that end. The area that is longer behind the heart is the rest of the body of the worm.
and it will be slightly more pointy than the head. This is the area where to cut from. I don't think it will grow back but it won't kill the worm.