the real scoop
Here's the real way to decide: take time. Go down to a good shop and sit and play some songs you know on both a concert and a tenor. Play for a long while. Then, go home and think about it a couple days. Go back to the store and try it again, or some friend's ukes. Think about it. Feel it. Listen.
Then, when you've found what size you like, take some time to try different brands: Kala, Ohana, Mainland, Lanikai, Applause, Bushman (my favorite) as many as you can find. Take your time and finally get one you like.
Otherwise, be spontaneous. Make a rash decision and maybe luck out or be like the rest of us and buy about twenty of all different sizes that all have good and bad qualities, and then start to worry about what different strings do. You will become a uke nut with a serious case of UAS. I believe Jake only a couple ukes.