Stretching a character is not a valid attack
This is related to the concept of called shots (see other questions here and here). The summary of those answers, and the relevant portion for this question, is that creatures are unable to make attacks that somehow subvert a target's hit point pool. I won't re-hash the excellent reasoning given in those answers, but I will add a few points of my own that I did not catch in a brief reading:
Hit points are not a direct representation of "health"; as per the PHB p.196,
Hit points represent a combination of physical and mental durability, the will to live, and luck.
In general, you cannot make an attack or cause damage that kills a player without reducing their hit points, because those remaining hit points represent the fact that they have resisted death from that effect.
For example, in your given scenario of a giant attempting to pull someone's head off, the fact that the player has not been reduced to zero hit points may represent the fact that they have enough energy to squirm free of the grasp, or to pull out their weapon and stab the giant's hand, dealing no damage but proving distracting enough to avoid dismemberment.
But perhaps you like the idea of pulling somebody's head off to kill them. This is a fantasy game, after all, and maybe you want dramatic moments. What to do?
The solution is simply to reflavor killing blows as whatever dramatic effect you want to achieve. If a PC is already dead, or you land a blow that fully kills them, then you can flavor that blow however you'd like- in this scenario, as the giant pulling the character's head off, or ripping them clean in two.
With that said, you should still be somewhat cautious about this. You should, for example, avoid destroying the PC's body entirely, unless an ability explicitly says so, as the state of a player character's body does have mechanical effect on how easy it will be to resurrect them.
The bulk of my answer has focused on the mention of dismembering by stretching; however, in the case that you wish to stretch a character without the effect of instant death but still causing damage- the closest official attack available to all creatures would be an unarmed strike (PHB p.195), which would deal bludgeoning damage equal to 1 + the attacker's Strength modifier.
A DM may adjudicate otherwise, of course, but there is no official guidance on damage taken from being stretched.