Generally, you'll look to make it something that does not stretch the string materials much, as that would mess with the player's intonation on a per-string basis (different thicknesses of the same material will stretch different amounts even if the distance to the fingerboard is identical). That would indicate a very low action would be necessary.
String height at the nut is also very important. I generally prefer my fretless basses to have a string height at the nut roughly equal to an open string's height over the first fret on a fretted bass. Or even better, as not every luthier cuts the same string groove, equal to the height over the second fret when held down at the first.
However, the lower you go at the 12th, the more compromises must be made. When you go too low, plucked harmonics just don't work and there usually just isn't enough relief in the neck to keep the string from buzzing at every position up to about the 9th. The string vibrates roughly equal amounts along its whole length between multiple nodes when you pluck a harmonic, so that means that if the neck/fingerboard doesn't have enough relief, you'll "fret-out" and make a ton of buzz against the fingerboard, no matter what harmonic you try to play. I realize this isn't a common playing technique for a bass uke, but if a player tries it (as I know *I* eventually would), you'd like it to work!
So, to recap: make sure you're not actually sitting on the fingerboard at the nut (give it .015" or so at least), make sure that there is enough relief in the neck so that harmonics don't buzz out, and from there, go as low as you can muster to keep from stretching out the player's intonation. If they want higher, they'll ask for it
I realize it's a uke and probably a whole lot shorter scale length, but the same principles still apply even at the smaller scale.
