There have been a few very large implementations of ERPNext. If I were to consult Lidl, we would have done a few low risk pilots, starting with their warehouses or back-office.
I would have gone for a federated architecture where each store / warehouse would have its own system that would then be consolidated at higher levels, rather than going for a mega instance with zillions of transaction rows.
There are many interesting things you can do when you have an open source system you can work to your advantage.
