Use mixture of various biomolecules and test them in UV. If they pass in the eluant and show some peak than it will confirm its size. Go for electron photomicrograph and measure the size on scale by analytical microscopy method.
Dialysis membranes and ultrafiltration membranes normally give the molecular weight cutoff (MWCO), assuming that the molecular weight is that of a globular protein. An estimate of the pore dimension could be obtained by looking up the molecular dimensions of globular proteins with the same molecular weight as the MWCO. Or you could ask the manufacturer.