What size ceiling joists and rafters should I use for a 14' span?
As @TheEvilGreebo mentions, 2x6 is not enough for a floor that will be used as "habitable space". I know because when I remodeled my attic to convert it to a bedroom, the existing 2x6 floor joists (over a very similar span) were not enough and I had to sister them all.

At a minimum you will need to check with your local (municipal or county) building department.  They should be able to tell you what you need.

It might be easier to get a structural engineer or architect to come out and tell you what to do.  If they are not drawing up plans you might be surprised that their prices are quite reasonable (they should be able to give you a quote for what it would cost for them to come out and generate a report up front).  It will also help when you get your building permit to be able to submit the report written by a certified professional to the building department.