The hardware to read the tapes are calibrated to the maximum size they are configured to accept. So when you hit up eBay, you will need to know the maximum amount of data you will need, and either the size of the largest tape drive to hold all that - if you are not getting a machine with an auto-loader - or the maximum number of drives that the machine’s autoloader can take, so you can size the tapes properly for the data.

Say you need to back up 25Tb. You are unlikely to ever need more backup than that. So you either look for a machine that takes 25Tb tapes, or you get a machine that can take max. 5Tb tapes, but has an autoloader that can hold at least 4 additional tapes (in addition to the one in the drive) such that all five will automagically cycle through the backup process. That way, all 25Tb will be backed up in either case without your direct and immediate involvement, all you have to do is rotate the tapes off-site after the backups are done, and slot the next ones in for the next backup run.

Obviously, incremental backups are a no-go, as backups are stored off-site. So it’s an all-or-nothing process. And as such, this is usually done both on your entire primary data set (for fast total-disaster restores) once in a while, with a different set of tapes focusing on your local/on-site warm backups and backing up only the atomic/incremental locally-stored backups for the day/week/month.