Node.js works on a principle called the event loop unlike other tradition web application frameworks and this is what helps make it extremely scalable for realtime apps. So Scale is not something you worry about when you choose NodeJS.
Node.js excels in real time applications because of WebSockets. WebSockets are open 2-way communication channels between the client and the server. And this makes it really easy to build realtime applications like Games, Chat Engines, etc.