Having touched last week on the issue of whether the shiur gadlus applies (12/13 to count as an adult) applies to a ben Noach, let me add the following: The Rambam (Hil Melachim 9:10) writes clearly that shiurim were only given to Bnei Yisrael and are not applicable to bnei Noach; therefore, a ben Noach is chayav for eating the smallest piece of eiver min hachai. Tosfos (Chulin 33) seems to disagree, as Tosfos writes that a b"n is chayav on a small piece of meat combined with bone and sinews (gid v'etzem). The implication is that one is not chayav for a small piece of meat alone, but only for a k'zayis -- however, that shiur k'zayis of eiver min hachai may be met by eating a combination of meat, bone, and sinew and need not consist of meat alone.
The Pri Megadim (Y.D. 62) suggests that the machlokes may depend on the definition of the concept of "achila". According to Tosfos, achila by definition means consuming a k'zayis. It is only the halacha l'Moshe m'Sinai or derasha of chatzi shiur which creates a new prohibition for eating less, but that does not apply to a ben Noach. According to the Rambam, achila by definition means consuming any amount. The din of chatzi shiur serves as an exemption for eating less than a k'zayis, but that exemption does not apply to the ben Noach.
(My son pointed out to me that the Sefer Tiferes Yosef quotes a few places that R' Yosef Engel discusses this same issue, e.g. Gilyonei haSha"s Yoma 74, Beis haOtzar 198 and a few other places.)