At less than critical mass, there are fewer "targets" for neutrons to strike, fission, and release more neutrons. As a result, the reaction dies out.

Critical Mass - Catholic rock - was created in 1996.

Critical Mass - Threshold album - was created on 2002-09-02.

We know that critical mass can be achieved in a nuclear reactor. If it could not, we'd not be able to get a chain reaction and generate heat to generate power. Reactors don't have critical mass when the control rods are in, but pull the rods, start the reactor up and away you go!

The critical mass depends on the shape; I don't think there is an upper limit to that.For the case of a sphere, the critical mass for U-235 is 52 kg. The corresponding diameter of the sphere is 17 cm.