FWIW, I have a cabin (okay, now have to officially call it a "shed"...) that is 12x20 with an additional 8x20 porch. We used it for quite a few years before building one with plumbing, etc.
If you are just using it for bunking the 12x16 should work. But if you are using it for a fridge, a table, a little closet and maybe a dresser, maybe a couple chairs etc, I would recommend going to 12x20. That extra 4' will make a big difference.
We didn't build ours, but there is no support in the middle and you can feel the flexing. It was built on 2x6's. I would recommend 2x8's and have a support in the middle. 12' is a fairly long span, and even my properly built 12'x24' deck has 2x12's.
If you don't have other buildings in the area (i.e. in-laws cabin/home, etc.) you might want to plan (but not build now) a porch for later. It is really nice to have a screened in porch, for the extra room for friends, and to get away from skeeters
A gable end porch would be real easy to add on later.
Good luck, have fun planning and building! (fyi my little place is in my avatar, but pic is a bit skewed - it shows the porch