  We derive an asymptotically accurate formula for the beam waist of
ultrashort, tightly-focused fundamental linearly-polarized, radially-polarized,
and azimuthally-polarized modes in free space. We compute the exact beam waist
via numerical cubature to ascertain the accuracy with which our formula
approximates the exact beam waist over a broad range of parameters of practical
interest. Based on this, we describe a method of choosing parameters in the
model given the beam waist and pulse duration of a laser pulse.
