What is the "maximum power" of an antenna?
It actually stands for the maximum power that you can push through this antenna.

This typically has three reasons:


When designing the antenna, there's some point at which the electrical field is maximal. You have to be sure that the electrical field still doesn't lead to dielectric breakdown (or excessive nonlinearities)
Safety: Power -> voltage -> safe to operate in the specified environment (electrocution risk); also: power -> power density -> safe to operate (hurting people in the main beam); this is usually more of a system than an antenna aspect.
Heat: No real-world antenna is 100% efficient. The remainder of energy is converted to heat, which increases antenna temperature, which might change or damage the antenna.


So, to evaluate these, you'd need to have a pretty detailed simulation of the antenna; typically, you'd want to actually test maximum power, not only simulate it, which will inevitably risk damaging the antenna you have.