Small if you are using a sub woofer. This will divert the lower end bass (basically below your crossover setting) to your sub and free up your M80s for the rest of the frequencies.

If you don't have a sub, set them to large so they can do full range, including bass.

If you set your M80 fronts to large and also have a sub, they both will do the lower bass frequencies which is not considered desirable by most people.

With great power comes Awesome irresponsibility.