I like to tickle small feet because they are dainty and cute, and they are usually attached to very giggly, high-energy women.
I also like to tickle a bair of bigger feet, because with the extra real-estate, there's more room to explore and take my time. Besides, there's something undeniably sexy about a girl with big feet who's rendered helpless by a little tickling.
WHen I was in high school, I used to know a girl who was VERY comfortable with having her feet tickled. She had big feet, and I LOVED slowly stroking my fingertips up and down her soles, making her snicker and giggle and try not to burst out laughing. When I'd catch her outside of school, though, it was a different story. 😀