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The Holmberg radius of a galaxy (RH or RHO) is a measure of its radius (along the semi-major axis) based upon surface brightness. It is specifically the radius to the region of the galaxy's surface where the surface brightness has an apparent magnitude of +26.5 per square arcsecond through a B filter. In other words, that size area of that amount of surface brightness would provide light equivalent to a star of such magnitude. Since surface brightness generally does not decline with distance, the radius generally indicates the portion with the same absolute brightness, no matter how distant the galaxy (possibly extremely-high-redshift galaxies don't follow this: I believe the surface-brightness-constant-with-distance characteristic is an aspect of Euclidean geometry).
A galaxy's surface brightness generally falls off fairly smoothly from its center, and an enclosing line in the celestial sphere or on the galaxy where its surface brightness has a particular value is termed an isophote. A galaxy's isophote marking the Holmberg radius is termed the Holmberg isophote.
An alternative galaxy size measure is the effective radius.