A smaller world keeps file sizes down and stops players from generating endless new chunks, which can hurt performance.

With server.properties

1. Open server.properties in the Files tab.
2. Find max-world-size. If it is not there, add it on a new line.
3. Set the radius in blocks, for example max-world-size=5000. The world border cannot be set larger than this.
4. Save and restart the server.

With the world border command

You can also set a border while in-game or from the console:

worldborder center 0 0
worldborder set 10000

worldborder set 10000 makes the border 10,000 blocks wide, so 5,000 blocks out from the center in each direction.

To avoid lag when players explore, consider pre-generating chunks with Chunky.