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We investigate the relationship of the HII region luminosity function (HIILF) to the HII region size distribution and density wave triggering ingrand-design spiral galaxies. We suggest that the differential nebular sizedistribution is described by a power law of slope ~ -4, with flattening atradii below ~ 130 pc. This contrasts with the conventional exponentialdescription, but it is physically and quantitatively consistent with thetypical observed value of -2 for the HII LF slope.  We have developed an interactive code that computes spatial isochrones forthe evolving loci of spiral density waves in disk galaxies. This allowscomparison of the nebular spatial distribution with the spatial isochrones forsimple rotation curve parameters. Our comparisons for four grand-designgalaxies suggest that the corotation radius r_co coincides with the outer endsof the star-forming arms. This value for r_co yields the best spatialcorrespondence between the HII regions and the isochrones, and also appears toyield a coincidence between the Inner Lindblad Resonance with the radial onsetof star formation in the arms. Thus, we suggest that isochrones offer a new,simple, and effective technique for determining r_co, and thus the spiralpattern speed. However, application of the isochrones also demonstrates thatevolution of the nebular population is difficult to spatially isolate in thesegalaxies.Comment: 15 pp, 8 figs, uses emulateapj. Accepted to A

HiiRegions in Spiral Galaxies: Size Distribution, Luminosity Function, and New Isochrone Diagnostics of Density-Wave Kinematics