<p>Observational data on quasiperiodic variations of the period derivative of pulsars are considered. Characteristic radii of the superfluid regions of pulsars are obtained under the assumption that these variations represent collective elastic oscillations of the superfluid vortex lattice (Tkachenko modes). The derived values of the radii were compared with those obtained from observations of the X-ray satellites NICER and XMM-Newton. These values were also compared with those inferred from the equation of state. The simple model of Tkachenko waves requires further refinement. This method can be used for approximate estimation of pulsar radii in certain cases.</p>