Re: not sure if this counts?
First off, congratulations on you sweet little baby! Glad they were able to see that she wasn't getting nutrients and were able to get her out in time!
I would just say exactly what you said in your post. She is small due to placental failure. She isn't a preemie, just small from not getting quite enough nutrients. I think most people would understand that.
I think that condition is called IUGR (Intrauterine Growth Restriction) so if you want a term, you could use that to explain to people.
SAHM to DD