  The local space density of galaxies as a function of their basic structural
parameters --like luminosity, surface brightness and scalesize-- is still
poorly known. Our poor knowledge is mainly the result of strong selection
biases against low surface brightness and small scalesize galaxies in any
optically selected sample. We show that in order to correct for selection
biases one has to obtain accurate surface photometry and distance estimates for
a large (>~1000) sample of galaxies. We derive bivariate space density
distributions in the (scalesize, surface brightness)-plane and the (luminosity,
scalesize)-plane for a sample of ~1000 local Sb-Sdm spiral galaxies. We present
a parameterization of these bivariate distributions, based on a Schechter type
luminosity function and a log-normal scalesize distribution at a given
luminosity. We show how surface brightness limits and (1+z)^4 cosmological
redshift dimming can influence interpretation of luminosity function
determinations and deep galaxy counts.
