First I calculated MB/journal on 4 InnoDB clusters and got:
0.625 -- what?
The first 3 numbers (and especially the first 2) were promising, but then I realized I would have to factor in account age.
So I calculated bytes/day/journal: (day == total sum of account ages in days across all journals on a cluster)
Much more consistent!
BTW, these sizes are considering all data and indexes on a single machine, but not redundant copies of data in the cluster, or backups.
So once somebody signs up, historically, on average, their account grows by about 1 kB per day. With 5.3 million users, that's 5.04 GiB/day. Considering redundant copies and backups, that's more like 20 GiB/day.
See why I hate disks?
Update: the data above is bogus, because our moving process has been moving active users onto InnoDB first, and active accounts are bigger. I'll post better numbers later.