The effects of sample size on the apparent properties of ferrites observed by microwave cavity perturbation methods are treated. Retardation effects associated with the propagation of electromagnetic waves in the specimen are considered. Results are given for the idealized cases of rods of infinite length and disks of infinite extent. Methods of adaptation to finite rods and disks are suggested. An approximate solution is given for the sphere which is in qualitative agreement with some aspects of the experimental observations. A formula relating the shift in resonance field to the sphere size is presented:HR−H0=4&pgr;M&egr;·(16&pgr;2/45)(R/&lgr;)2.