*macOS 10.13 High Sierra Beta* Cannot Resize APFS Volumes on External Hard Drive
I'm no APFS expert but my understanding is that once the whole HD is formatted as APFS it becomes a "container" that can hold multiple volumes.  You can add volumes (partitions) to the container but they all share the entire space of the container and resize themselves dynamically.  Therefore there is no option to resize a partition or volume in disk utility. The file system takes care of it automatically.