My first important question is for what reason are you requiring the zeta potential of fibers? The Zetasizer measures mobility (movement) under an electrical field as the measured parameter as, and you would guess, fibers do not move like spheres.... It may be better to report this mobility parameter rather than a conversion to zeta potential that will rely on assumptions such as spherical 'particles'. In this way there is no ambiguity as to what is measured as opposed to derived. In the 1930's there was work (Perrin) on the conversion of mobility of cylinders to zeta potential. See slide 15 in:
November 10th 2010 Effect of shape on rheological properties