Resolved - GELI version 5 (in 8.2RC2) doesn't attach a provider with an incorrect provider size and there is no force option.
Unfortunately none of the FreeBSD boot disks (including the LiveFS) have GELI available to attach to a provider so I had to reinstall 8.1 (fortunately it was a fresh install of 8.2 so no big deal), attach/mount the drive, copy the data from it to another file system and re-initialize the provider to fix the size issue (using the 'geli resize' command returned errors).