This is the key. I'd argue that you can keep a 20k codebase in your head. If the codebase is larger than that, you need to be confident that you can safely ignore the rest of it while working on your piece.
If you need to manually check with the rest of the codebase before making changes to the chunk at hand, you're lost.
If you need to manually check with the rest of the codebase before making changes to the chunk at hand, you're lost.