For a ceiling with a clear span of 16', doug fir 2x8 would be a minimum if you want storage above, remember you are also hanging ceiling material and insulation from these joists, 2x6 unsupported won't do. 2x10's would be a solid choice and might even get you 40psf live load. If you put at least a 1/2 rigid continuous under your ceiling material and a min. of R30 above, you'll be quite comfortable.