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.
The wet side low voltage floating cord will be the limititation, you also want a power supply side cord that is short enough that the power supply box can not be knocked into the pool while it is still plugged in (think running kids snagging the low voltage floating cord), so the use of an extension cord on the supply side is quesitonable depending on if you have it safely located. (I think the instrucitons on mine say to keep the power supply box at least 10 feet from the edge of the pool, this is not practical where my outlet is.)