I bought a foam one that is oval and shaped like an O. I was told to avoid the inflatable ones because they could lose air, the U shaped ones could spread when you sit on them but the oval shaped ones offer support, won't spread or lose air.
I found that if I removed the cloth cover, I had support but not directly on the surgical area which made all the difference.
It is about 18"x12" maybe by 2-3" thick. Worth every cent when I used it!
Love,
Clare