question

First, yes, just let us know what server instance type you'd like for your Title ID, and we'll set that up. We can change that whenever, but please be aware that any servers already running will remain running (so you may need to terminate some instances, or at least disable a build to move to the new instance type), and the way the service is configured, you'll be using the same instance type across any active builds.

As to how to determine the best size of instance type, since every game's logic is unique, what you'll need to do is test your server running with no players, with a minimum of players and (if possible) with the max number of players. If your max is hundreds or thousands, you would need to write a tool for simulating the connections from that many users if you want to have complete coverage. I'd recommend running these tests on a local machine, so that you have full access to it and can immediately debug any issues that you see. Log all the information on used/available CPU, memory, and disk activity. That way, you can use that data to approximate what size server you need. You could also then do a test in your own AWS account in EC2, so that you have access to the full dashboard of info on the server's resource usage.