Safek in shiur (amount)

We once before touched on Tosfos's difficult to understand opinion (chullin 65, brachos 26) that even though safek derabbanan l'kula, when one has a safek in shiur (amount), the rule of safek l'chumra applies. (See: http://divreichaim.blogspot.com/2005/12/sfeikos-in-shiurim.html). This Tosfos should be applicable to manay halachos, e.g. can one rely on a smaller shiur of k'zayis for a miztva derabbanan like maror on Pesach, or a smaller shiur of revi'is for kiddush on Shabbos morning. I have not seen Tosfos addressed, so was happy to find tucked away in the Ma'adanei Yom Tov on the Rosh in brachos 14: The Rosh paskens that t'ima (tasting a liquid) on a ta'anis up to and including a revi'is is OK, but no more than that. Asks the MY"T, why up to and including a revi'is - according to Tosfos "kol shiurei chachamim l'hachmir" means ad v'lo ad b'chlal (up to but not including...) even by dinei derabbanan, so only up to but not including a revi'is should be included in the heter? He ultimately ends with a tzarich iyun.