It all depends on the visitors you are expecting.
For starters 5$ , 512Mb RAM server is enough, I'm using that for one of my project on express . The application is currently receiving 20-30 real-time and approx 5-8k users daily .. and on this load the CPU hardly goes above 5-8% and same for memory consumption.
You can easily handle 200-400 realtime on 512Mb RAM server.
So, i think now you have pretty good idea how much memory / cpu a node application can consume.
PS : On Digital Ocean i think you can scale the infrastructure without effecting the application itself.