My experience is with copilot and it uses various models, but the sweet spot is between 60 and 120 lines. With psuedo xml tags between sections

Might be different across platforms due to how stuff is setup though.

My AGENTS.md is 845 lines and it only started getting good once it got that long. I'm still wanting to add much more... I'm thinking maybe I need a folder of short doc files and an index in AGENTS.md describing the different doc files and when to use them instead.

I know copilot supports nested agent files per folder.