This is primarily directed to Rav Moshe. I've seen a number of times that you refer to an os gedola as the shiur for kuba/zanav. I'm aware of Rav Elyashiv's opinion that the shiur is an os ketana (width of a Vav/Yud) and The Shevet Halevi (chelek 2 siman 155) writes that the shiur is a word (which word?) and then he says, "and less than a full word tzarich le'ayen ledina, nevertheless the width of a Vav or Yud certainly doesn't pasel". (Based on this, more than a Vav/Yud (which is less than an os gedola) may be the shiur.) What is the source for an os gedola?