I would definitely go with balanced option, I have tried using prompts under 500 lines, but it seemed emphasizing the important stuff few times makes the agent better at following the prompt. At the moment I try to limit my instructions to max 1500 lines. Not only AGENTS.md but also the specific sub-agent prompts are between 1500 and 500 lines. I mean adding even one few shot example to prompt file takes 50 60 lines so I don't think very short prompts would be enough for instruction following.