On the Theoretical Period-Radius Relation of Classical Cepheids
  We present the results of a comprehensive theoretical investigation on the Period-Radius (PR) relation of classical Cepheids based on new sequences of full amplitude, nonlinear convective models constructed by adopting a wide range of both stellar masses and chemical compositions. For log P in the range 0.9 - 1.8 a very good agreement is found between theoretical predictions and current available data whereas outside this range, both at shorter and at longer periods, nonlinear radii attain intermediate values between empirical relations based on different Baade-Wesselink (BW) methods and photometric bandpasses. 
Bono Giuseppe, Caputo Filippina and Marconi Marcella