. Standard photometric dimensions of 37 galaxies are derived from recent detailed photometric studies. The relations of photometric dimensions to the laws of luminosity distribution in galaxies of different types are discussed. Micrometric dimensions of over 1200 Shapley-Ames galaxies (to be published later) have been derived from 13 main series of photographic determinations, including 6 series of new data. The relations between systems of micrometric diameters D and axial ratios R = d/D are determined and the relations between photometric and micrometric dimen- sions are discussed. A provisional reduction of 10 series to the (corrected) system of the Heidelberg survey by Reinmuth was made for the Shapley-Ames objects. The probable errors of the reduced values from individual series are of the order of 12 per cent for both D and R. Strong correlations between apparent diameter and axial ratio, i.e., tilt, and galactic latitude are indicated. The re- duced or "face-on" diameters D (o) of galaxies of a given type are also closely correlated with total magnitude mT and can be used as distance indicators. An application is made to the calibration in terms of the current distance scale of the relative distances of several clouds and clusters of bright southern galaxies observed in the Mount Stromlo survey. The application of the corrected mean diameters and revised luminosity laws to the determination of improved incom- pleteness corrections and total (or asymptotic) magnitudes is briefly discussed.