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 the optical spectra of 216 GRQs from the Sloan Digital Sky Survey (SDSS). As a result, we obtained the composite spectrum covering the wavelength range from 1400 Å to 7000 Å. We calculated the power-law spectral slope for the GRQ's composite, obtaining α λ = -1.25, and compared it with that of the smaller-sized radio quasars, as well as with the quasar composite spectrum obtained for a 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 bins. Moreover, we found that there is a flattening of the continuum with the increase in the projected linear size of the radio quasar. We show that α λ 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 the X-CIGALE code to compare the consistency of results obtained in the optical part of the electromagnetic spectrum with broadband emission. The parameters obtained from the SED fitting confirmed the larger dust luminosity for the redder optical continuum.