As an American, I pronounce the name Anastasia as a four-syllable name with emphasis on the third syllable: anne-ah-STAY-sha. But this isn't the accurate Russian pronunciation of the name. It's a horrible circle: I can't stand the American pronunciation, but I can't train myself to say it any other way.
For Russians, Anastasia becomes a five-syllable name with emphasis on the fourth: ah-nah-stah-SEE-ya. And in Russian, it's written Анастасия.
Thanks to my favorite names website, behindthename.com