first of all, don’t fixated on regular script (楷書), read and think on bronze script or seal script; please.
look at the components semantic tree (部件樹):
矩 is 大 + 人 + 工
in the same page, the 漢語多功能字庫 stated clearly that
roughly, the “大” separated from “工”, only the “手” is connect to “工”; which make the character “巨”
this picture shown such separation :)
巨 has lost the left hand side of 工, and so naturally the radical appears to be "匚".
Baidu (obviously not authoritative) and Pleco say it's 匚
only in regular script, that 巨 would appear belong to radical 匚 (radical 22), or 匸 (radical 23).
巨 in bronze script:
巨 in seal script:
these three clearly shown the “工“ in the character “巨”
Is there a fairly authoritative, widely accepted source on which radical 巨 falls under?
as droooze said, the authoritative dictionary is 康熙字典
another one using 540 radicals is 說文解字. you may guess it, “巨” is in volume 5, (工, radical 147)
last, authoritative resources for etymology:
use the 漢語多功能字庫 first, compare the “pictures” of different scripts. read the etymologies (形義通解); these’re experts’ explanations in one place.
imo, it’s enough for novices, even advance users.
if one need a bigger pictures of characters in particular scripts, shift to 小學堂
have fun :)