Like others above, I'd recommend starting on a concert size, tuned gCEA.
That said, the most versatile size (for tunings) is the baritone. With commercially-available strings, you can tune it gCEA (standard 'ukulele tuning), GCEA (linear tuning), DGBE (conventional baritone tuning, like the four treble strings of a guitar), and dGBE (re-entrant bari tuning). That covers the most common uke tunings you'll come across. I remember the late Dirk Wormhoudt of Southcoast saying if he could have only one uke, it would be a bari, for this reason.