I've seen various sizes, I guess it depends on a show's budget and how paint they want to use for it. I would imagine that film cels are by larger than TV shows due to the need for more detail.
Anime these days (bar Sazae-san apparently) are all made on computer these days so cels are a bit of a dying market. :cry:
*disclaimer: this post is mainly guesswork, I've seena couple, but have never owned a cel myself!*
