The size of RF cavities in particle accelerators is determined by several factors, including the RF frequency, beam pulse shape, and physical space constraints within the facility. Safety is a critical concern, as high-voltage components pose significant risks, necessitating proper insulation and distance during operation. The discussion emphasizes the importance of adhering to safety protocols and the need for a controlled environment, such as a Faraday cage, to mitigate hazards. There are concerns about the honesty of individuals seeking advice on building accelerators without fully disclosing their intentions or understanding the associated dangers. Overall, the conversation highlights the complexity of designing RF cavities and the paramount importance of safety in accelerator projects.