women's clothes sizes, small, medium or large?

I'm buying a coat for my Mom, the one she has, has a tag, with a #8 inside.

I showed her several coats on J.C. Penney web site, and will buy the one she liked best.

However, they list the sizes as Petites, or misses. small, medium or large.

Mom is 5'3" tall, about 140 lbs. She is in Assisted Living, so it's difficult for me to take her shopping.

Any idea, of which of those listed sizes, equals a size 8.

Any help will be appreciated.

thank you in advance/