$R_{\rm e}$, or not $R_{\rm e}$: Developing $R_5\equiv R_{-2}$ as a scale radius for galaxy sizes, masses, and mass-to-light ratios

The effective half-light radius $R_e$ marks an arbitrary 50-per-cent light boundary, and scaling relations involving such `effective' radii and their associated surface brightnesses, $μ_e$, systematically (and undesirably) vary as the percentage changes. Here, the projected radius $R_5\equiv R_{-2}$