[colechristensen]

The answer is complicated. ZFS wasn't designed with a small number of heterogeneous drive sizes in mind.

If you don't care about redundancy then you can span a volume across as many odd drives as you want free and clear.

If you do care about redundancy, you need sets of similar drives put together and then it only makes sense if you have a large number of drives.

Raidz has performance penalties and should only be used with a large number of vdevs unless you care little about speed.

An array of mirrors is the best way to go for small to medium sized zfs installations which means pairs of same-sized drives and only 50% available space.