I'm with Paul on this one - Lighter wind = longer handles for me.
I use 3 different sets - 13", 14", 15" for different wind conditions. The 14" are my "standard handles", probably used 75% of the time. They are a good compromise set - a little longer, but not too long, for almost all my usual flying. 13" work great in screaming wind and I'm on my X/V, no need for all the extra leverage the long provide. Shorter give me more control. Light winds see the 15" come out. I just feel more comfortable with them in light winds. Not sure if the "technical" term is leverage, but I feel a bit more lift, a bit more control, with the longer in those conditions.
If you're wondering how I have 14", since the way they are measured from the factory is before bending and Rev only offers handles in odd inch sizes (13", 15", etc), I had mine made for me. Same gent that makes JB's dogstake, made them. I had him bend them a little flatter (less), more like my Indoor handles. Gives me a bit extra brake already built in! Gotten so used to them that picking up stock handles with the rings, takes a bit to readjust my inputs! And that is my roundabout way to say it is all about developing your own personal "feel"!