Gillian Anderson - No. She's about 5'3" and maybe 105 lbs. soaking wet.
Kate Winslet - At one time, yes. Before she got pregnant, she admitted to being a size 12-14, which she was told was plus sized. She always said she was happy with her body, but after she had her baby, she went on a diet to try to get down to a size 8 because she said she was having trouble getting roles because of her weight.
Other examples:
Liv Tyler - her sister is already a plus-sized model for Lane Bryant, but Liv herself is getting close. There was a story I read that said she's about a size 12 now - often considered plus size.
Camryn Manheim - she's openly said she's a size 22.
Emme - a working model who's a size 16.
I'll see if I can find pics of these women.