Q. Al netilas yadayim is recited when eating an egg’s volume of bread; less than that, one washes without a bracha. What if one is not sure how much bread he will eat?

A. Indeed, Shulchan Aruch (O.H. 158: 2) mentions the opinion of the Rokeach that if one eats less than an egg’s volume, he should perform netilas yodaim without a brocho. Mishna Berura (ibid. 9) explains that since less than that amount the food is not contaminated by tumah, our sages may not have ordained netilas yodaim. It would seem from the Mishna Berura that he agrees to the ruling of the Shulcha Aruch and so he mentions in s. 486: 1.
However, Igrois Moshe (O.H. 4: 41) quotes the opinion of HaGra (ibid. 7) that on a k’zais bread you also recite netilas yodaim and rules that although on the onset one should certainly eat an egg’s volume, if he doesn’t want to eat more than a k’zais he also recites netilas yodaim. He also seems to require a “hachlata” before washing hands as to how much one is planning to eat to avoid doubtful blessings.
Otzros Chayim (Tikun Seuda 4) mentions that if he thought that he would eat a beitza volume and washed with a brocho, and then realizes that he cannot eat more than a kezais, it is not considered a brocho lebatalah.
Nishmas Shabbos (434) cites the opinion of the Ritva, that if someone washes hands, recites a brocho and then changes his mind and decides not to eat, it is not a brocho in vain, (and accordingly rules Chazon Ish 25: 9) . He combines this view with the opinion of the HaGra above, and states that if one only consumed a kezais, one does not have to eat more if he doesn’t want. He further adds quoting Shevet Hakehosi (5: 39) that if someone is in doubt whether he will be able to eat an egg’s volume, as is common on a winter’s third seuda, one is allowed to wash with a brocho, since this constitutes a sfek sfeika or double doubt to be lenient.
Horav Shlomo Miller’s Shlit’a opinion is also that if he is at least sure that he will eat a kezais but is in doubt of eating more, he can wash with a brocho. He mentioned that in reality the kezais amount isn’t much (aprox. 28 gms.) and there are also lesser shiur opinions, that could be joined to be lenient when in need.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a