Building a shelf header

Question;
I am building a shelf in my garage. It will be approx. 26 inches deep and 10 feet wide. It will be secured on the back to the wall and on the right to the wall the left will be bolted to an adjustable rack the front will be free floating. I need to know the proper header beam i would need to cover the 10 foot span. I was thinking 2 – 2×6’s would be enough but reading different things leads me to think i would need 2 – 2×10’s. The shelf may a descent amount of weight on it after filled with storage stuff.
So what is the proper header beam I would need to build?