According to this joist span table at 16" o.c. you can span a 2x8 12'-5".
I can not imagine that you could not safely get by spanning 2x6s at 16" o.c. spacing over 7'.
That would be a very surprising difference. If you look at the drop in span distances when you go from using a 2x10 to a 2x8 its like 3 or 4'. But if you really need and exact answer you will have to look up a different joist span table.
May 14, 2010 Rating
2 X 6 joists by: Mike in NC
The real answer depends on your local building code. The practical answer can probably be best answered with another question.
Why would you want to use 2 X 6 joists for any span? I highly recommend using 2 X 8 joists for most applications, and 2 X 10's if the span is greater than 10 feet. That assumes 16" on center joists. If you use 12" OC, you may get by with longer spans, but I still would never use less than a 2 X 8.
I am building a 10'x12'deck (not attached to a ledger board). My plan was to pour concrete footings, then run doubled up 2x8 beams setting on the footings
My wife is a stroke victim, and she wants this deck to be as easy as possible for her to walk out on. She would like it to be as high as possible, with