  We present the composite optical spectrum for the largest sample of giant
radio quasars (GRQs). They represent a rare subclass of radio quasars due to
their large projected linear sizes of radio structures, which exceed 0.7 Mpc.
To construct the composite spectrum, we combined 216 GRQ's optical spectra from
Sloan Digital Sky Survey (SDSS). As a result, we obtained the composite
spectrum covering the wavelength range from 1400 {\AA} to 7000 {\AA}. We
calculated the power-law spectral slope for GRQ's composite, obtaining
$\alpha_{\lambda}=-1.25$ and compared it with that of the smaller-sized radio
quasars, as well as with the quasar composite spectrum obtained for large
sample of SDSS quasars. We obtained that the GRQ's continuum is flatter
(redder) than the continuum of comparison quasar samples. We also show that the
continuum slope depends on core and total radio luminosity at 1.4 GHz, being
steeper for higher radio luminosity bin. Moreover, we found the flattening of
the continuum with an increase of the projected linear size of radio quasar. We
show that $\alpha_{\lambda}$ is orientation-dependent, being steeper for a
higher radio core-to-lobe flux density ratio which is consistent with AGN
unified model predictions. For two GRQs, we fit the spectral energy
distribution using X-CIGALE code to compare the consistency of results obtained
in the optical part of the electromagnetic spectrum with broad-band emission.
The parameters obtained from the SED fitting confirmed the larger dust
luminosity for the redder optical continuum.
