Shiur of Bracha Rishona:[1]

There is no required measurement of food that must be consumed for the recital of a before blessing prior to eating or drinking, and thus even a mere taste of a minute amount of food requires a blessing to be said beforehand.[2] [This however only applies if one swallows the food, as explained next.] Only regarding an after blessing was a measurement of consumption required.

Birchas Hamotzi:[3] The blessing of Hamotzi has no measurement just as is the law by all before blessings, and thus even if one only desires to eat a mere crumb of bread which is less than a Kezayis, nevertheless he needs to say Hamotzi beforehand.[4] The Shiur of a Kezayis was only given regarding Birchas Hamazon, and other after blessings and not by a before blessing. [Nevertheless, due to other reasons, some Poskim[5] rule that during a meal one is to initially eat a Kezayis of bread immediately after the blessing. Practically, we do not suspect for this opinion.[6]]

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[1] Seder 8/10; Luach 2/11; Admur 167/4 and 168/7 regarding Hamotzi; Michaber 210/1; Rambam Brachos 1/2, 3/12; Rif Brachos 27b; Tosafus Brachos 39a; Rosh Brachos 6/16

[2] The reason: As it is forbidden to benefit from this world any amount without saying a blessing. [Admur 168/7, Seder and Luach ibid]

[3] Admur 168/7 and 167/4; Seder 8/10 and Luach 2/11 regarding Bracha Rishona; Michaber 168/9; 210/1; Yerushalmi Brachos 7/1; Rambam Brachos 3/12

[4] The reason: As it is forbidden to benefit from this world any amount without saying a blessing. [Admur 168/7, Seder and Luach ibid]

[5] 1^st opinion in Admur 167/4; 1^st opinion in Yerushalmi Brachos 6/1; 1^st opinion in Rokeiach 329; Implication of Rama 167/1 who says one must distribute a Kezayis to each eater; M”A 167/7 learns from Rama that it is an obligation to eat a Kezayis and M”A concludes that one is initially to be stringent like this opinion; Bach 475; Olas Tamid 167/11; Arizal in Shaar Hamitzvos Parshas Eikev; Yifei Laleiv 167/2; M”B 167/35 concludes it is proper to follow this opinion; Vetzaruch Iyun if Admur learns this opinion to require a Kezayis in all situations, or only when eating with others.

Answer of contradiction-Background and explanation: See Rama 167/1 that one must distribute a Kezayis to each eater and M”A 167/7 learns from here that it is an obligation to eat a Kezayis. Admur 167/4 rules that according to this opinion, the Kezayis must all be eaten in one shot. The Machatzis Hashekel 167/16 learns from here that one is not to talk until he swallows a full Kezayis. This may imply that Hamotzi may only be said over a Kezayis of bread, in clear contradiction to the ruling here that Hamotzi may be said on any amount. The Degul Merivava addresses this question and states that the intent of this ruling is not to say that perhaps only a Kezayis requires a blessing of Hamotzi, but rather that perhaps in order for the blessing of Hamotzi to cover an entire meal, one must eat a Kezayis of bread. On this the Machatzis Hashekel ibid states that it must initially be eaten without interval. So also explains Shulchan Hatahor 167/4 and 177/1; Kaf Hachaim 167/15. Practically, Admur a) implies that the entire discussion of Rama 167/1 is only regarding laws of Derech Eretz and not Brachos, hence avoiding any contradiction between the two places and b) Admur brings 2^nd opinion that does not require a Kezayis and concludes that so is the custom.

[6] Admur 176/4 rules the custom is like the 2^nd opinion; Ketzos Hashulchan 37 footnote 22