When eating by a host on Shabbos one is occasionally faced with a situation where the guest has more exacting standards of observance than the host.
What should one do when eating by a host on Shabbos and being given a small piece of challah after hamotzi (which is just barely a kezayis according to some opinions or may be less than a kezayis) and the host is not willing to cut more challah? (When I asked for more challah, he pointed to tiny slices on the challah board and was unwilling to cut a normal piece.) Should one rely on the opinion that what he ate was a kezayis (since there are opinions that a kezayis is very small) and bentch? Or what?
You are pretty talented if you can actually tell on a piece of challah if it is 27 gram or 33 gram, but if you are sure that it was 27 you can definitely bentch. It is hard to imagine that your host gave you a piece of challah that was so small that even after you ate the other piece that it wasn’t this amount. (Meaning that both pieces together were smaller that the size of a golf ball, of an Isreal box of matches).
The opinion of R’ Eliyashiv and R’ S. Z. Auerbach Zt”l brought in V’zos Habracha siman 1, that one may rely on the smaller amount to make a bracha achrona. As a side point see Kovetz Hamoadim 3-pg. 465 in the name of R’ Chaim Kanievsky, that the Steipler, who usually says to use the larger shiur, that you can rely on the small shiur of kzayis, which would be 17 cc, which is a third of an egg, when you don’t have a choice. This shiur is what sick person or someone that can’t eat matzoh should eat to be yotza matzoh, and they also have to bentch afterwards. Your situation wouldn’t be worse then that.