AM Transmitter Power
There are so many variables that a good answer for this question cannot be had. At night, our local station at 930KHz which can normally be heard for over 60 miles, can barely be picked up a few miles away. On the other hand, a 50KW station may be able to reach 100 miles in the daytime and "skip" thousands of miles at night due to ionospheric conditions.
For normal daytime operation, I would think that 5 to 10 watts would be sufficient. The antenna is probably the most critical element in the entire system, for no matter how much power the transmitter has, if the signal can't "get out", all that power is wasted.
Ultimately, the power and operation of any transmitter is a legal issue, regulated by your nation's communications regulations.
Dean