Unless height is an issue, why limit yourself to a 6" deep timber? It's the height that's going to give the beam its rigidity. Why not look at an 8" or 9" deep floor joists? We use 9" x 2" and even 9" x 3" floor joists all the time.
Although, the last bridge we made (over a stream to a rough old fishing pond so aesthetics wasn't an issue) we used 'open metal web floor joists' because we had a couple lying around. Super rigid and super easy to 'feed' over to the other side being so light.
Like this...Image may be NSFW.
Clik here to view.
