The values depend on the leakage inductance (Le) of the transformer. Off the top of my head select C such that the resonant frequency of the LC circuit is 10 times that of the switching frequency. Fres=(1/2*pi)*1/(sqrt(L*C)). Select R=sqrt(L/C); it might be 2*sqrt(l/c)
A more elaborate method includes finding out the characteristic RLC values of the circuit (no snubber), and then using simulation software to find out a RLC network capable of providing critical damping (single cycle with no oscillation) with minimum losses. Series inductance in snubbers is actually useful as long as it's not too far from the optimum value.