Building a Mezzanine Office 2x10s or 2x122
Assuming ordinary loads on the floor and etc. you can look this up on a span table, such as this one.  

  

This shows that for a 16' span, 2x10's spaced 12" on center are adequate even with the Eastern spruce number for a 40#/ft-ft live load.  

You could get by with 2x8's if you use #1 grade yellow pine and only need to support a 30#/ft-ft live load, but I'd much rather leave some room for error than try to squeak by.  

With 2x12's, you can actually get by with 24" spacing, although that's going to be a springy floor, it's probably a significant savings.  Spruce will make it, but you could probably upgrade from spruce to #1 or #2 yellow pine for some room for error and still come out ahead.