Lots of folks (including me) will say they would prefer something a little shorter, if nothing else than to reduce the cost of the darned cables (and I'll admit to that second part). But you already have enough of the answer above - use the heaviest gauge you can and don't worry about it (think BlueJeans 10AWG or something like that). If you don't have identical lengths that's also no biggie - electricity propagates at approx 1ft/nanosecond and you can figure out the rest (as in, it just will not matter).
Speaker cable length
I have Cary tube amps and Harbeth speakers and I am kind of limited as to the placement of my components in my listening room. The optimum positioning for best sound would require a 27 foot speaker cable on one side and an 18 foot cable on the opposite side. Are these runs to long and could they harm my equipment or somehow lower sound quality?