My apologies for the delayed answer. The transmit power is, by default, automatically set using the Meraki auto power algorithm. This algorithm attempts to find the best operating wireless channel for client devices. Changes to transmit power and channel can be performed manually from within the mobile app.
The coverage area for the GR10 is approximately 500 square feet, but your mileage may vary depending on the environment. The same antennas are in the GR60, given similar propagation and distance properties.
Datasheets for each access point can be found here: