New Systempartition-Backup-Strategy-External Drive

Until now i have saved systempartition eighter as file on the same and second hdd. and also-after duplicating drive-to its partition.
Now i want to save it to an external drive-nonbootable.
1. should i partition this drive? is 2tb enough or better getting 3TB then? i would avoid 4TB since then i would fill it and if defective a lot of data lost. ok should be duplicated then.

have one esata-slot on my old PC. and esata-pci-slot. only not possible booting from the latter.
Thanks for any hint.
Already have found one:
https://www.partitionwizard.com/clon...-strategy.html