I have the same one at home, from spencer's and everything... it's beautiful.... I know that I'm allergic to most metals and the pentacle itself didn't bother me but i had to switch the chain.
not sure on size sorry :( ... it may be like 18 in or 20, i know im strange with necklaces, it cant be too close to my neck or i feel like im sufforcating lol...but i had no prob with this one :)
i think maybe cause the chain moves more so it irritates allergic reaction more? i like to switch all my chains in general lol...i actually like using like black cord lol )O( Blessed Be