If one eats only a kzayit of bread, does one wash with a bracha and recite birkat hamazon? Also, if one eats a very small amount of bread, does he wash beforehand (and with a bracha?)

For Hamotzi: any amount requires a Brachah. (Shulchan Aruch OC 168; 9) For Netilat Yadayim: Less than Kazayit – no need for Netilah More than Kabeitza – Netilah with Brachah Between Kazayit and Kabeitza – Netilah without a Brachah (ibid 158)