How do you manage your codebase once it gets large?
intellectronica: One important thing to notice as a codebase gets large is that it's going to slow you down. Accept that, and pay more attention to maintaining a high quality codebase. Invest in reviews of new merges, refactoring of stale code and documentation. Automate as much as you can - make sure you can create good API docs, and that there are tools for working with the existing codebase. Finally, try to remove code as much as you can (refactoring, or simply deleting code you no longer use).