I agree about NOT coiling the wire. Consider a Z/fanfold instead. As far as lengths you may want to go to a home depot and pick up 42 ft of cheapest wire you can find. That way you can cut 2 18ft and 1 6ft length and try that to see if you hear a difference. Also consider if you do get a high end cable of different lengths resale will be tough.
Odd length speaker cables
Given the considerable cost of most high end speaker cables and corner equipment placement in a non dedicated room I am in need of 18 foot run and a 5-6 foot run. How much detriment coiling 12 feet of cable on one side will have for signal path versus difference if one obtains 2 cables of different length?