Hi,
You will likely see very different numbers between the one-time memory usage at build-time, and the ongoing usage at run-time. You may well need 2GB to build the image, however that should be fine as either you build locally (most computers will have at least that much), or use a remote builder provided by Fly. Not sure what spec the latest ones have but they should be pretty big e.g
Once your app has a built image, then the question is how much RAM it uses to run. As @rubys says, a basic Nodejs app will run on even the smallest instance (comes with 256MB). You can monitor your usage in the metrics.