Shorter cranks don't reduce your power, they decimate your power. Unless you are a trained cyclist that is used to cadences up to and exceeding 100rpm. Most regular cyclists find cadences over 80RPM mentally fatiguing even if not physically so. As to frames properly designed to use short cranks... which would those be? All production frames assume the use of cranks considered "normal". That would be around 165mm on the short end and 175mm on the long end. Not that stops anyone from using shorter or longer cranks as they see fit. As I understand it, a slack seat tube, set back seat post configuration emphasizes low rpm torque, and such a configuration assumes the use of longer, not shorter crank lengths. The ultimate and polar opposite configuration would be a track bike. If you can describe a lower and more aerodynamic position than a 75* seat tube angle track bike with a -15* stem and Pista bars I'd like to see it. Swap out the Pista bars for Triatholon bars and you can maintain that low, aero position more or less comfortably for hours.