The plural morpheme in English is regularly represented by the allomorphs [s], [z] and [iz]. The rule is as follows:
(i) kicks [kiks];
(ii) if the morph ends in one of the voiced phonemes (including the vowels), /s/ is represented by [z] (cats [kats]);
(iii) if the morph representing the noun morpheme with which /s/ is combined to form the plural ends with a sibilant ([s], [z] etc.), /s/ is represented by [iz] (sizes [saiziz]).
Lyons, John. 1968. Introduction to Theoretical Linguistics. Cambridge: Cambridge University Press.