How does 巨 resemble a hand (彐) holding a carpenter's square (工)?
The forms you're looking at are actually the ancestors of「矩」.「巨」was formed by cutting off the person.



The analysis starts from this form

西周金伯矩盉蓋集成9412

which depicts a person「大」holding a tool「工」.


  Carpenter's square is much too specific, and I would think that it is an over-interpretation.



Exaggerate the person's hand to emphasise that the tool was held:

西周金伯矩卣集成5228
Swap out「大」with the similar-meaning「夫」(man), and permanently detach part of the hand onto the tool:

西周金裘衛盉集成9456


The left hand side of the above character should now heavily resemble「巨」, which is created from omitting「大」or「夫」.

秦簡睡・語5　篆工部說文解字今楷　


  「夫」was later semantically corrupted into「矢」(arrow > length, measurement), forming「矩」.
  
  東漢隸桐柏廟碑　今楷　




References:


季旭昇《說文新證》
小學堂
國學大師


中國社會科學院考古研究所《殷周金文集成》