A note on Schultz In Schultz1959 (https://doi.org/10.1103%2Fphysrev.116.526), a Feynman polaron radius is defined from the Gaussian form of the Simple Harmonic Oscillator specified by the Feynman solution to the polaron problem.
I rederived some of these results (and the key thing - figured out that a 0.44 ‘magic number’ was actually 2/9, i.e. 0.4 recurring), and decided to put the info out there somewhere. The core I’ve written up as LaTeX here: https://github.