If you create a zpool with '-o autoexpand=on' you can replace disks with larger ones one at a time, waiting for each one to resilver. After replacing the last disk the pool will jump to the new size.
What difficulty did you encounter? I've had a pool running since ~2009, originally as 4x750GB, then 4x3TB in 2013, and I recall that the transition was excellent. As oofabz said, the trick is to enable "autoexpand", which can be done even after creation. In fact, I recall that I had replaced each drive, initially seeing no increase in capacity, but after enabling autoexpand the new space was immediately available.
You can grow it horizontally by adding a new vdev. ZFS doesn't allow you to do anything you may ever possibly want to do, but it does allow you to do more than any other RAID solution. It's not perfect but it's still pretty impressive.
At first, I assumed that RAIDZ was the obvious way to go. But I switched to a concatenation of mirrored pairs; I grow the array by adding another pair to it.
I've actually had six disks fail in six months, without data loss. Was scary. But wow.
