From left to right, projected density maps and radial profiles of stellar mass, stellar luminosity, and gas star formation rate for a simulated spiral galaxy of stellar mass 3.8 × 10¹⁰ M_⊙ selected from the recalibrated simulation. The vertical dashed, solid, and dot-dashed lines mark the positions of R₅₀, R₁, and R_29, g, respectively. For this star forming galaxy, R₁ is a better measure of the size of the galaxy for its proximity to the edge (truncation radius) of the disc. The edge of the disc coincides with a net drop in the star formation rate surface density, the starting hypothesis for the new definition of galaxy size.