How do I choose beam size?

Could you please explain how I should approach selecting the appropriate beam size? What factors should I consider in making this decision? Is there a specific formula or rule of thumb that I could follow? Also, how does beam size affect the overall performance and efficiency of the system? Would a larger beam size necessarily be better, or could it have its own drawbacks? I'm interested in understanding the balance between precision and coverage that needs to be struck when choosing a beam size. Could you provide some practical examples or scenarios where different beam sizes might be suitable? Thank you for your help in clarifying this important aspect of the process.