you said you got 171's with warranty...were the ones that got warrantied 171's? if they were then you wont be able to "trade" but if they weren't then you should get the same length that you originally had.
im 5'6", 145lbs, and use 171s. i dont think they will really hurt you, deff dont go any shorter, but you might want to jump up to a 179.
if you can do doubles, then shouldnt you know what size you need anyway?