Whats the likely sag factor to be for a 2 x 3 inch beam. Outdoors, very little load bearing (apart from wind, couple of plastic sheets)
12 feet length in bog standard pine. My options are. Leave as is, put in a central post so span would be 6 feet either side (but prefer not to) or a diagonal brace at each end which would reduce the central span to say 8 feet.