How to set worldborder on Minecraft server?

Worldborder limits the area players can access. The number given in the java command is the width instead of the radius.

Step by step

Plan in which world and in which center the border will be located. Check the coordinates of player bases.
Authorized calculation set the center with /worldborder center 0 0 and the sample width with /worldborder set 4000 in the target world.
If scaling down is necessary, give the players time. Evaluate the structures and portal exits that will be excluded in advance.
Observe the border at about 2000 blocks from the center and try Nether or separate world behavior separately.

Drawing borders does not delete old chunk files. Disk cleaning is a separate process.