Go for a full sized one. I have a full sized one for my 18" saddle (piccy in PG if you want to see) and it's to small really, I could do with an extra large one!
If you're going for Le Mieux the medium size fits a 17" saddle well.
Large is for 17.5" + however I don't think it would be too big and I would prefer it to be a little bigger as it could shrink a little bit in the wash.