what is the max wattage of an unlicensed, uncharted, pirate fm radio station?

.250 W (roughly the equivalent of an FM transmitter for your car, or a Raspberry Pi, broadcast range 10-100 ft). Pirate FM generally wouldn't care about the rules, and don't usually last long on the air, before they get shutdown and equipment confiscated, and in jail. The music licensing companies (BMI, ASCAP, PPL, PRS, etc) would be more interested in shutting them down, because of lost revenues, than the government, unless they were stepping on a local station's signal.

In order to get decent range, your antenna would be fairly visible, and obvious. Building the tower without construction permits would usually invite local zoning representatives to look into what your were doing.