Isophotal Major-Axis Diameters
Redman  suggested that galaxy diameters should be operationally defined by the µB(B) = 25.0 mag/arcsec 2 isophote. This corresponds to a surface brightness about one-tenth of the night-sky brightness and is very nearly the maximum detectable diameter on blue prints of the POSS (=  General Catalogs of Non-Setllar Objects). Holmberg , however, chose to define his diameters at a few percent of the sky brightness, µpg 26.5 mag/arcsec2. The choice is arbitrary, but as fainter isophotal diameters are chosen the constraints on the data become more demanding and, as a result, the diameters in general are probably less well defined.
Major sources for diameters of galaxies are [75, 138, 140, 186, 223, 246, 247, 224, 270] and others as listed in Table 8 of RC1 (=  in Secondary Galaxy Catalogs) and Table 5 of RC3. In addition, diameters are listed in the Atlas de Galaxias Australes (=  in Illustrations Supporting Classification Schemes) UGC (=  in Primary Galaxy Catalogs; but see also [109) UGCA (=  in Primary Galaxy Catalogs) MCG (=  in Primary Galaxy Catalogs; but see also [109) ESO (= [27; but see also [109, 205] in General Catalogs of Non-Stellar Objects) ESO-LV (=  in Secondary Galaxy Catalogs) SGC (=  in Primary Galaxy Catalogs) VCC (=  in Primary Galaxy Catalogs) and KUG (= [182, 183] in The Kisu Survey) which in turn have been transformed to a standard diameter, D25, in RC2 and RC3 ( in Secondary Galaxy Catalogs). Standard diameters for 237 galaxies measured at 25 and 26.5 mag/square arcsec , revised and up-dated to include 608 standard photometric diameters  as well as many transformation equations for other studies, based on [39, 45, 66, 194, 211] cluster spirals ; see also Kallogyan's study [155, 156, 157].
Axial Ratios and ``Face-On'' Diameters
The measured apparent major-axis diameter is a function of inclination [75, 76, 139, 278]. Galaxies viewed more edge-on have larger apparent diameters due to finite thickness of the disk resulting in a greater optical path: and thus an increased surface brightness at fixed radius. Corrections to a ``face-on'' isophotal diameter D(0)25 have been investigated , revised  and applied to galaxies in the RC2 (=  in Secondary Galaxy Catalog). However, see also [266, 267]. Axial ratios, necessary for correcting the diameters, are taken principally from the MCG ( in Primary Galaxy Catalogs), UGC ( in Primary Galaxy Catalogs) or , and are transformed to the standard 25 mag/arcsec2 isophotal ratio, R25, in RC2 ( in Secondary Galaxy Catalogs).
Mean Surface Brightness
The RC2 ( in Secondary Galaxy Catalogs) tabulates two average surface brightnesses: M'25, the average surface brightness contained within an ellipse of diameter D25 and axial ratio R25, and M'e, the average surface brightness within an aperture Ae that contains half of the total light (i.e., containing half of the asymptotic magnitude Btotal ). The first definition is relatively straightforward to compute, the latter is more complicated and somewhat model dependent. See also the mean surface brightness study of isolated and double galaxies .