World size is more about disk space. With a VPS you should be able to keep an eye on disk space and increase as needed. It also depends on how often you want to run backups.
Ram is for how many players you have online at once. That should be plenty for 10-20 people, but depends on what version and if mindcraft is modded or not.
The key is monitoring. But you could limit to 10,000 blocks to start with and increase it later.
