I'm not familiar with the storage system you use, but usually these storage systems have multiple controllers which can be owners of LUNs presented to the hosts. With using only a single LUN/datastore you are actually "wasting" performance, so you may consider to present another LUN with the second storage controller as the LUN owner and create a second datastore for your VMs.
Regarding datasore resizing. With ESXi 5.5 there's no need anymore to use extents, i.e. present another LUN and add it to the existing datastore to increase its size. You can simply increase the LUN size up to ~64TB, rescan the storage adapters on your ESXi hosts and then increase the datastore size from its properties window.
André