I think the ideal woman for me would be the most politely termed "Rubenesque" (though I do prefer them a bit more "in shape" than Rubens painted them). He showcased the beauty of the female body and all of its imperfections perfectly (IMO).
The only time that I have actually had sex with a woman (am married to a man but have always liked woman as well) was when I was much, much thinner, I'd say around 160ish, and she was probably 200-250 lbs. I still found her quite beautiful, despite the fact that I find myself almost repulsive when I pass the 200 mark (don't even ask how I felt when I found out I'd passed THREE O.O).
As for general attraction though, Id say Rubenesque describes it perfectly for me. Plump and curvy, yet very pleasing to the eye and touch. I'm just not a fan of rail thin. I love boobs and butts too much to sacrifice them for a flat tummy.
Of course all of those things are purely physical. I have not had the pleasure of having an actual relationship with a woman, so I am not very well versed on what I like in a woman personality wise. I know that with men and women both, I find intelligence, confidence, strength, a sense of humor, and a general joie de vivre to be very enticing. If I found those qualities in a rubanesque woman I would probably fall head-over-heels!
Edited to say:
I do have to note that the one exeption to not finding rail-thin attractive for me would be Audrey Hepburn. I find her to be just gorgeous, though a lot of that could be in the way she carried herself.