The distance you can achieve using any microwave radio link depends on the path length, frequency in use, transmitter power, antenna gain and receiver sensitivity.
In the UK, maximum power allowed in the Unlicensed and Lightly Licensed spectrum are set by the regulator. These are denoted in "EiRP". This can be fairly easily explained using an analogy with a light bulb. If you imagine that a torch bulb radiates 1 Watt of light energy equally in all directions (not realistically possible because of the metal on the bottom of the bulb), this is what's known as an "Isotropic Source" and that's what the "i" stands for in EiRP. If you then put the torch bulb into a torch reflector, it will focus much of the energy into one direction. The amount of light energy radiated in that direction will be increased. The amount by which the energy is increased will be in proportion to the size of the reflector.
A larger reflector will be capable of focussing the energy into a finer beam, meaning the concentration of energy within the beam is higher and therefore capable of reaching a further distance. The amount by which the energy is concentrated is known as the "Effective Radiated Power" or ERP. Where this is being compared to an "Isotropic Source" it is denoted EiRP and measured in Watts.
The following are the power limits set in the UK for Unlicensed and Lightly Licensed Outdoor Radio links:
Frequency Maximum Power (EiRP)
The power radiated from the antenna diminishes exponentially with distance (known as Free Space Path Loss). However, a larger antenna at the receiver end will be capable of capturing more of the signal, therefore the antenna size/gain at both ends is important. Manufacturers publish the receiver sensitivity figures for their equipment, therefore, by knowing the transmitter power, antenna gain, distance and receiver sensitivity you can calculate how far you will be able to transmit your signal. It is good practice to add an additional "Fade Margin". This means budgeting for a higher signal than you need at the receiver, in order to account for environmental variations.