I saw in Wikipedia that WASP-12 is a star with spectral type of G0V while they said its temperature is 6360 K. Does that mean it must be a star with spectral type of F7V or F6V? Its size is not like a normal G-type star should have.
There is actual variation in the literature.
The WASP team announcement of the discovery of a planet states "the host star is a supersolar metallicity ([M/H] = 0.3+0.05–0.15), late-F (Teff= 6300+200–100 K) star which is evolving off the zero-age main sequence."
Simbad, which is a "catalog of catalogues" lists the star as G0V, and cites Bergfors et.al. "Stellar companions to exoplanet host stars" Which indeed stats G0 V as the spectral type.
Finally Wikipedia cites Collins et al "Transit time measurements of WASP-12b..." to get the temperature of 6360+130-140 K.
There is an inconsistency between the papers. Part of this may be due to WASP-12 being an evolved star and moving away from the zero age main sequence. Also its metallicity will have an effect on its effective temperature. But part of the variation may simply be different scientists working with different data.