Setting the view distance on a Minecraft server
Like on your client, the server also has a View Distance. This is the maximum of how far a player can see on your server. So if a player has their client set to 12, and your server is set to 10. They still won't be able to see past those 10 chunks.
By default your server will be set to 10. This is just what Minecraft has servers default to. To change this we're going to head to Server Configuration under Game Tools. Here we're looking for VIEW-DISTANCE. If you can't find it press CTRL and F and search for VIEW-DISTANCE.
You can set this value to whatever you want. This will add more stress to your server, so we don't recommend you set it too high, as it will cause lag issues.
A chunk is 16 x 16 blocks.