Floor joist weight cap
While you do not have enough loading information to give a more exact answer, you could typically use taller and/or larger flange I-joist or 4x2 floor trusses.  The key is height to whatever you put in.

I-joist can easily give you the span you need, its just a matter of loading. 

For comparable cost at longer spans and higher loadings, floor trusses can pretty much do what ever you want if you give them enough height.  Just locate a truss company and meet with them and they should be able to guide you on a proper truss design and get you a quote.

For a barn, I would probably go with I-joist but if you need to run any mechanical or plumbing, I would recommend floor trusses.