  Perfect (optical) vortex (PV) beams are fields which are mooted to be
independent of the orbital angular momentum (OAM) they carry. To date, the best
experimental approximation of these modes is obtained from passing
Bessel-Gaussian beams through a Fourier lens. However, the OAM-dependent width
of these quasi-PVs is not precisely known and is often understated. We address
this here by deriving and experimentally confirming an explicit analytic
expression for the second moment width of quasi-PVs. We show that the width
scales in proportion to $\sqrt{\ell}$ in the best case, the same as most
"regular" vortex modes albeit with a much smaller proportionality constant. Our
work will be of interest to the large community who seek to use such structured
light fields in various applications, including optical trapping, tweezing and
communications.
