Is this trivially small? Don't bother licensing. People will just copy and paste regardless of what I do and I don't care. But it gives you the option to change your mind later. Licenses are irrevocable. (I specifically mean not applying a license and maintaining all rights.)
Is it a small part of a whole that people need to use? LGPL. But take this with a grain of salt. FSF says to prefer GPL.
Otherwise, AGPL. If there was a more strict but also commonly used license I'd use it instead.