Without seeing the guitar, it's hard to know--mediums are a place to start, but you could end up using heavys--if it seems counterintuitive, simply think about the idea that, regardless of neck length, you're tuning to the same pitch.

The tension on a high E string is going to be greater on a longer neck, so you lighten the string gauge to create less tension. The inverse would logically be true--the shorter the neck, the less tension needed to hold the same high E, so to keep the strings from becoming flaccid, they need to be heavier.