For a small building, should joists or girders run the long dimension?
I'm not sure exactly what the orientation of this floor is? It is also common to split the 16 foot span in half with a center carrier beam, thus allowing you to use 8 foot joists on either side. The advantage to using two sets of 8 foot joists as opposed to single longer ones is that you can use smaller less expensive stock, and you can more easily control any wain or bowing typical in longer stock. Using a center beam also gives you a good structure to add additional center concrete post supports. This is also done by running a perpendicular double beam half way under the 16 foot joists, which gives the same support as using two 8 foot joists with a center support.  

Another consideration is what kind of material and orientation you want to use as decking. If you are using planking, the aesthetics may be a determining factor. Got any drawings we can look at?