Are you talking about theoretical RF power, or total system power, which is dependent on additional parameters, for instance, the silicon used.
If RF energy, what assumptions are you making, and what parameters are known. Antenna gain and directivity, receiver sensitivity, transmitting environment, and so on. Coming to grips with RF propagation is fraught, normally you have to allow for a wide margin of error, and do measurement with the proposed system configuration. BER (Bit Error Rate)is the most common parameter for describing a data link, no matter what the communication channel is.
Generally, you would buy modules for WiFi, to avoid issues around meeting local RF propagation regulations. Additionally, rolling your own PCBs working at the chip level is not recommended, although some manufacturers do supply layout guides, and if it is a commercial product, certification of your production device will be required.
Modules have the ability to control RF power output in software, but generally WiFi is used for greater distances than you propose, and their power consumption is quite high in comparison to say bluetooth, which is designed for close in lower power operation.
In either case, the specs you are interested in are defined at the module level, since they usually have a PCB chip antenna.
There are also propriety ICs, made by the likes of Nordic Semiconductors, that work well in this sort of application, alongside the bluetooth ICs they make,
cheers,
Richard
