I would go with a Medium size. It may be a bit large, but you can put some extra substrate or balled up newspaper in it to fill in some of the space if needed. The Smalls are really small and in a few months, your snake will outgrow them.
I totally agree with this. The smalls are reeaally small and you can always fill up the space in the mediums with crumpled newspaper or sphagum moss....