backup device size

The obvious answer would be at least 256GB. But there are some industry standards in portable or external hard drives.

The next biggest size is typically 500GB. That's what I would get. The next intervals are usually 1TB and 2TB (terabytes, or 1,000GB).

Here are some options from the Apple Store:

Storage - Mac Accessories - Apple

sbreiden wrote:

How much portable drive space is needed to backup a 256GB hard drive?

Thank you.

This article includes the size backup drive you need:

Back up your Mac with Time Machine - Apple Support

"Ideally, your backup disk should have at least twice the storage capacity of every disk or volume you're backing up. If your backup disk doesn't have enough storage space to contain a complete backup, Time Machine will let you know."