The Build server takes a very different approach to multiplayer Minecraft, wherein everyone has creative mode and is free to build whatever they wish. The benefit to doing this on a server, rather than in single player, is everyone else can see you're builds, and you can take inspiration from other builds as well.
The plugin acts a little bit differently in this server, since everyone is in creative mode. You have almost infinite claim blocks from the start, and you must claim an area before being allowed to build. Don't worry too much about the size of the claim, as you can expand a claim later if you want to make your creation larger.