Doesn't the area described by a turbine's motion scale with the square of the blade-length, so given a circular area covered by a turbine, the power will scale linearly with that area?
Obviously, cost scales more than linearly with blade length but it’s a bit like big O - the n^2 factor dominates. This is why wind turbines have been getting bigger and bigger. And why the cost of domestic or small-scale wind turbines remains stubbornly high despite the dramatic fall in the average cost per MW seen for wind turbines - as the falls are largely driven by the ability to manufacture larger and larger turbine blades. While falls in costs for solar PV can be seen at every scale.