Mostly I fantasize about women; they can be anything from slender (but still with some hips) to what I think you (Eugler) would call "chubby".
I prefer the term "curvy" for a woman who's softer/rounder than "athletic" but not all the way to "chubby" (for which I prefer the term "thick", or as Gunsmoke says, Rubenesque). For me, with women, it's all about the hip/waist ratio. My absolute favorite (in fantasy) is a tall, curvy woman, with a sassy and dominant personality.
When I fantasize about men, they're athletic. Swimmer's physique, as Incendiare said, or a bit more muscular is good, too. With men, for me, it's all about shoulder/waist ratio. My favorite fantasy guy is a bit (or a lot) taller and slightly more muscular (and stronger) than I am, again with a dominant and aggressive personality.
This is a fun topic.