Hello!
I'll tell you what I've told some other members, hope it helps.
IMO, EF doesn't run any "bigger" than a traditional brand, they simply cut for a woman's body and go up to larger sizes. I've been wearing their pieces for ten years, and they recently changed some of their items from PP, PS, XS, S etc. to numbers. To figure it all out, I suggest trying on some pieces at full price. If that's not possible, here’s the link to their size chart on their site:
http://www.eileenfisher.com/Ei.....cale=en_US
As an example, I am a size 4/6 and 36B, and I wear a PS, XS, or a 4 in their pieces, depending – although in some jackets I prefer a S.
Hope this helps a little!