Actually, the expression is "as skinny as a beanpole." Beanpoles are slender, but may be quite tall. "Little" and "small" do not fit the image."
Also, "skinny / thin as a rail." I think this one goes back to the rail fence, not the iron rail as used by the railroads. (Abe Lincoln was a rail-splitter, and was also as skinny as a rail.)