Stentor is the Homeric herald with the proverbially loud, "brazen" voice, equivalent to that of fifty ordinary men: Homer, Iliad 5.785-6 (web address 1); OCD4 s.v.
The present headword phrase is presumably quoted from somewhere. (Its only actual attestation outside the Suda occurs, in the accusative case, in the thirteenth-century Byzantine historian Georgius Acropolites.) In any event it makes explicit what is left implicit in a famous passage in Aristotle (Politics 7.1326b7: web address 2). Expressing there his insistence that a polis should not be too large, Aristotle rhetorically asks who could be its herald if he was "not Stentorian" (mh\ *stento/reios).
See also e.g. Lucian, de luctu 15.