I am still working out some details but I think you are probably
the Greek letter nu.
"A New Riddle"
"New" and "nu" sound alike.
I come from the hell as you know
Dunno what that's about. ... Ah, it turns out to be a pun on "Hellas" = Greece, as revealed by Ankoganit in comments.
but the nun gave birth to me
"Nun" is the name of the letter of semitic abjads (e.g., Phoenician) corresponding to "N", and Greek nu is derived from it. (As is our letter N.)
I can be big or small.
The Greek alphabet, like ours, has upper and lower cases.
I am at the unlucky position.
Thirteenth letter of the Greek alphabet.
When two of me are here, you see envy.
I think "envy" = "NV" here. Perhaps what we have is one capital and one lowercase nu; a capital nu looks like a latin N but a lowercase nu looks more like a latin v.
I can say how free it is, or maybe, how frequent it is.
In statistics nu denotes number of degrees of freedom; in physics, frequency.
Sometimes, I just pass by, as a little mystery, indifferent to anything that tries to attract me.
Nu is the symbol used for neutrinos, whose interactions with other particles are notoriously weak.
Often, I am ytisned;
Nu is the usual symbol for "specific volume", which equals the reciprocal of density.
or, I may be the right proportion of poison.
I fear this is a reference to "Poisson's ratio".