Is there any average value for the Alfven radius of main-sequence stars? if there are no such values, then how may I calculate it??
There is no average value. The large scale magnetic fields of main sequence stars are both a function of spectral type and rotation. In turn, rotation is age-dependent. In addition, the gas pressure at a distance from the star depends on conditions in the stellar wind and this too will depend on spectral type, rotation and age.
There are no universally agreed models for the behaviour of these quantities.