Specifically, as a DM, what is your ruling on if one were a Shaman who ended up in contact with the Elder Mythos by his wandering spirit and then decided to summon up/call up on Elder Mythos creature like say a Wendigo or a Star Spawn of Cthulhu? Does that mean said caster has to eat the entire body of a creature or just a part of it for the Wendigo? What about sacrificed humanoid?
This is a bit confusing, as your text asks different questions from your title, but I'll give it a go:
Do Giants count as sentient creatures for material components?
Yes, as long as the giants were sentient.
Does that mean said caster has to eat the entire body of a creature or just a part of it for the Wendigo?
I don't see anything saying that you have to devour all of the flesh of the creature(s), only that you must devour the creature(s) flesh. Otherwise it'd be pretty untenable as a spell without some sort of gluttonous magic.
What about sacrificed humanoid?
That one says you need the sacrificed humanoid; presumably, that's the entire thing, often sacrificed as part of or just prior to casting the spell. In other words, no double dipping on the same sacrifice.