The Arukh haShulchan (OC 363:34) contrasts the ammah used for measuring for an eiruv, and the ammah used for measuring the maximum 4 amos at which carrying an object is prohibited.
One may carry in public on Shabbos less than four amos distance. When measuring those 4 amos, a person built according to the norms for human beings should measure out their own amos -- 4 personal forearms.
But because an eiruv has to be sharable, we use a standardized ammah. (Although in the previous se'ifim he discusses the fact that even with a given opinion of what an ammah should be in modern measures, this is still a range with a "stingy ammah" or "generous ammah" to be used as appropriate. Humans having only human abilities to measure, Hashem gave Moshe a range, not a specific value.)
When speaking of fasting on Yom Kippur (OC 612:8), the Arukh haShulchan rules that when we speak of drinking less than "melo lugmav -- the fill of his cheeks" (eg a sick person who must drink but is able to minimize the violation) is indeed his own cheeks.
However, when it comes to food, the measure for Yom Kippur is a kekoseves hagasah (the volume of a particular breed of date, a "fat" date). Or in your case of birkhas hamazon ("bentching") and most other mitzvos, the unit is a kezayis (olive volume). These would be measured using standard measures. Which makes sense, since even an individual doesn't have a connection to any one particular olive. It's not like an ammah, which refers to a forearm, and a person has only two (one hopes).
So, it would seem the kezayis of bread would stay the same, but the other measures.... Well, remember I wrote about the person being anatomically typical? For example, a thalidomide baby wouldn't have incredibly short amos. Maybe being shrunk would qualify. Maybe not, since everything is still to scale.
But there is another problem with the kezayis. Beyond the kezayis, there is the measure of sevi'ah -- being satiated. For example, by Torah law, birkhas hamazon is only in the context of "ואכלת ושבעת וברכת - you shall eat, you shall be satiated, and you shall bless." We held ourselves to a much stricter standard, and bless G-d for even just a kezayis of food.
But that is assuming that a kezayis is much less than sevi'ah. Your hypothetical shrunk person will be satiated with far less and therefore may be biblically obligated way before reaching the rabbinic limit. And perhaps on Yom Kippur too one can't eat to satiation if someone would have far less than a kekoseves of capacity.
Needless to say, the Arukh haShulchan doesn't discuss your case. I guess the guy was just.... overlooked?