First, you need to establish what kind of an area you need to cover. What range will my fm transmitter have, how much power do I need? We are all limited by laws of physics. Range is thus limited by several factors: Optical visibility.
This can sometimes be up to 40 miles, if you are looking out from a mountain top. Interference from other stations on the same or close-by frequency. Receivers are not ideal and are even becoming more crappy in this modern age of crappy chinese dollar radios. Such receivers have difficulty discerning your signal while there are other strong signals close by. Transmission power. Even if optical visibility is 20 miles, 1W probably won't get you more than a mile. If 50 Watt ERP is used, it's very likely that 20 miles of range will be achieved.
This is because 50 Watt ERP is ample power to propagate a strong signal 20 miles. If 1 Million Watts of power is used, it is very likely that signal will only propagate just over 20 miles. This is because the range is limited as described in point a) above(optical visibility). Assuming the antenna has a clear view, the frequency is clear and an average (poor) quality portable receiver is used, typical transmission power vs range figures are as follows:
It is occasionally possible due to special atmospheric conditions, such as inversion etc. Such special conditions happen rarely and only last for a very short time so it is not possible to rely upon them in any way.
In order to cover so many square miles it is necessary to setup a grid of transmitters and link them via wireless audio links, making sure they do not transmit at the same frequency as they could interfere.