We have performed a propagation-of-errors analysis on two methods used to determine the spatial parameters of a laser beam. We measured diameters of a diode laser beam focused by a 993 mm focal length lens. Measurement uncertainties of less than 1% can result in uncertainties greater than 200% in locating the beam waist of the laser. We compare the inherent uncertainties in the spatial parameters as obtained by the two methods. Longer focal length lenses and lens position can reduce this magnification of uncertainty, but would require large propagation distances.