Method 1:
- Login to your control panel.
- Enter your configuration editor, click text editor next to serverconfig.txt
- Find the 'autocreate' line and change to the value you want. Values are: 1 for small, 2 for medium, and 3 for large.
- Save and apply.
- Restart your server.
Method 2:
Login to your control panel.
- Enter your configuration editor, click configuration editor next to serverconfig.txt
- Ensure the auto create check box is checked
- Change the world size dropdown to the size you want.
- Save and apply.
- Restart your server.
Note: You will need to generate a new world for this to work, for help on how to generate one click here.