  The low $Q^2$ slopes of the the transition form factors provide a unique
method to measure the sizes of the neutral pseudo-scalar mesons, since they do
not have electromagnetic form factors. From the slope one obtains the "axial
transition RMS radius" $ R_{PS,A} = \sqrt{<r^2>}$ for each PS meson. The
present status of theory and experiment for these quantities are presented. A
comparison of the $ R_{PS,A}$ is presented along with the electromagnetic and
scalar radii of the $\pi^{\pm}$ mesons and the proton. We observe the striking
similarity of the values of axial transition radii of all of the pseudoscalar
mesons to each other and to the charge radius of the $\pi^{\pm}$. In the $Q^2$
= 0 limit the transition form factor is a measure of the pseudo-scalar meson
radiative width (lifetime) and is a possible fourth (unexploited) method to
perform such a measurement. The $\pi^{0} \rightarrow \gamma \gamma$ decay rate
is a test of QCD at the confinement scale. There is a firm QCD prediction with
a theoretical uncertainty of $\simeq $ 1 \% which calls for an experimental
test at the same level of accuracy. There are three methods that have been
utilized to perform this measurement and the present status of the experimental
tests are outlined. The current accuracy is significantly less than the
theoretical uncertainty. The efforts to improve this are briefly summarized.
