Vivaldi Antenna: size & max input power & software simulation
The size of the Vivaldi will determine the lowest frequency that it works at.

The input power will probably be limited by the coaxial cable. With a high power cable on a small antenna, you might find that the PCB overheats in the feed region.

There is modelling software but it's not cheap ($10,000 -  $100,000), and not easy to learn. Try a demo of FEKO, it converts to a free Lite version which is limited but could just about model a basic planar Vivaldi.