How can I find the radius of a exoplanet using doppler spectroscopy I have the stellar and planet mass, planet velocity, radial velocity of star, stellar semimajor axis.
How should I go about to find the radius of this exoplanet?
In general, you can't. The spectroscopy tells you about the properties of the star and the properties of the orbit and gives some information about the mass of the planet ($M \sin i$, where $i$ is the orbital inclination).
If the spectroscopy were super-precise and with extremely high signal-to-noise and high time resolution, then you might be able to map the Rossiter-McLaughlin effect with such precision that you can estimate the planet radius. However, this would only work if the planet is transiting in front of the parent star - in which case you might as well have used transit photometry in the first place.
Another possibility is to search in the spectra for the Doppler-shifted scattered starlight from the planetary atmosphere (e.g. Martins et al. 2015). However the amplitude of this is tiny depends on both the radius of the planet and its albedo/atmospheric composition.