windmill power goes something like the 4th power of the blade speed. As a result, your maximumepower is harvested at the windmill blade tips. To increase the efficiency, you want maximum possible tip speed, but wear is a function of shaft speed. so you want high tip speed, low shaft speed. Therefore you need a large area.
Or lets put it in terms of the disk plane. Harvestable wind is a function of the area of the intersected disk. If you double the radius, you quadruple the harvestable wind. Actually, you do better than that because you reach higher (with a higher wind speed), and farther from the tower (which slows the wind). So again, you want a large radius blade.And yes, long blades under extreme torsional and bending moments, at high speed IS a recipe for blade failure.