You're exactly right as far as I know. You would have to build a new raidz with larger drives or more drives, and with both old and new pools online, zfs send -> zfs receive all the data from old to new; then you could remove the old raidz and throw away the ridiculous tiny obsolete drives.
BTW, a bit of terminology. Zpool is the top level (root) ZFS structure for *any* use of ZFS, even one which only uses a single drive (degenerate case - which actually "works" just fine and dandy, and is a great improvement over ext4fs because of numerous ZFS features such as snapshots and block checksumming). You can have any number of independent zpools. Within a zpool you have vdev components.
From the bottom up, a vdev can be a single drive or partition, or multiple sub vdevs in the form of either (1) a logical concatenation of vdevs (like RAID0), a mirror (sort of like RAID1 but better), a raidz (single parity; sort of like RAID5 but considerably better), a raidz2 (double parity; sort of like RAID6 but considerably better), or a raidz3 (triple parity, beyond RAID6).
A zpool is then either a single vdev (possibly nothing but a single drive or partition), or multiple vdevs combined in exactly the same way.
Thus you can have a tree of arbitrary complexity, for example a mirror of mirrors of mirrors of mirrors of ...
Or a raidz of raidzs. Or a mirror of raidz2s. Or a raidz3 of mirrors. Or, you name it.
Finally, within a zpool you can create, in a completely ad hoc manner, any combination of zfs filesets (recursively if you wish). This is orthogonal to structure of vdevs and sub vdevs. Each such fileset can independently grow to any size, limited only by the size of the zpool it lies within.
And the stark reality. Once you create a raidz[23], the number of components and the utilized size of the components are forever set in stone for that particular raidz[23]. Mirrors, on the other hand, can have extra elements added (from 2-way to 3-way to arbitrarily many copies). And, if you know how, even the sizes of the individual elements can be grown.