I think the general rule of thumb is that 2/3rds of a PCX lens is useable - look into spherical aberration. I've found that a 63mm biconvex lens is the minimum. You might be able to go smaller with an aspheric lens.
EDIT: This is what will happen if your lens is too small. The corners of the beam set will begin to flare outwards and won't converge at the focal length. i.e. you'll lose power
