  (Abridged) This paper presents an exploration of two fundamental scaling
relations of spiral galaxies, the luminosity-rotation speed (or Tully-Fisher;
TF) relation, and the size-luminosity (SL) relation, and the dependences of
their scatter, at red and IR bands. We verify that the observed virial
relations of disk galaxies are given by Vobs \prop L^0.31 and Rd \prop L^0.33
using distance-redshift surveys of high surface brightness (HSB) and low
surface brightness (LSB) non-interacting galaxies. Thus the galaxy surface
brightness (SB) is Sigma \prop Rd \prop Vobs \prop Mvir^0.33. We derive the
first extensive J-band TF and SL relations using 2MASS data products. The TF
residuals are fully independent of SB and other tested observables. TF
relations (TFRs) for various morphological types have different zero points,
but a common slope, such that early-type (redder) disks rotate faster than
later-types (bluer) of the same luminosity. This dependence of the TFR is a
direct consequence of the more fundamental dependence on color, which itself is
related to the galaxy star formation history. The SL relation scatter is mostly
dominated by SB, but color also plays a small role. The TF and SL residuals for
HSB and LSB galaxies are weakly correlated, in agreement with the earlier claim
by Courteau & Rix (1999). This suggests that spiral disks of all color,
brightness, and barredness are, on average, dominated by dark matter even in
their inner parts. The observed scaling relations of disk galaxies are likely
the result of the simplest scenario of galaxy formation, but their color
dependence remains a challenge to hierarchical structure formation models. The
scaling relations and their derivatives are contrasted against galaxy formation
models in our companion paper.
