Balona (1977) has described a modification of the Wesselink technique of radius determination which takes into account the effects of observational errors using the Principle of Maximum Likelihood. This leads to a tighter period - radius relationship for Cepheids than obtained by the classical method and suggests that some of the problems associated with radii estimation of RR Lyraes may be due to the incorrect statistical treatment of the data. With this in mind, we have applied the Maximum Likelihood technique to well-observed RR Lyraes. Except for two stars, the data used were taken from the literature.