Ok, here are my thoughts:
Sizing is going to depend on couple of things:
1 cord length / outlet location, I have an older Dolphin Diagnostic robot in my 20x40 ft pool, I have it plugged into an outlet near the shallow end corner, this model was offered with a 50 or 60 ft cord, mine has the 60 ft. which is just long enough to allow it to clean the opposing corner of the pool with the power supply sitting about 6 feet from the edge of the pool. If mine had the 50 ft cord, I would have to place the power supply midway along one side of the pool for it to reach.
2, How much stuff gets into your pool, and partly how long the robot runs on a single cycle.