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The best way to determine this is with real world testing. First find out what the baseline is (what your existing app uses with no users online) and then under load. That difference should allow you to get an idea of the load put on your server for each user it is serving. With this is would be possible to do a rough calculation of the RAM needed to support your planned app.
Otherwise, off the top of my head and based on what you’re currently running I would recommend starting with the 1GB droplet size ($10) and monitor it’s resource usage. If you find you need to upgrade you can resize your droplet from the control panel to a larger size.
A few tools that will come in handy:
top - shows running processes on your server and their resource usage along with the current overall load. (you can exit top with ctrl+c)
free -m - give you an overview of your available/used memory.