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.Are there any focusing lenses on Aliexpress or eBay that can be used with the NUBM31T?
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