You pronounce
Anastasia the exact way I do. I briefly considered it while pg with my dd,
Anna. Ultimately, I decided it was too long for my use and I do think there would be something of an issue with pronunciation as everyone around me seems to say ann-a-STAY-sha. You have to decide whether the name is worth it to you. If you love it enough, it will be. BTW,
Marisol Anastasia is gorgeous.
Marisol has really grown on me in the last year or so (along with
Maribel).
I'd like to add that you'd stand a better chance of having the name pronounced the way you prefer in a large, urban area.