You'll need an external drive with at least of space, though 128GB or more is recommended for a smooth experience. How to use an external storage device as a Mac startup disk

Plug your drive into any compatible Mac and see your desktop exactly as you left it.

Modern Macs have security settings that block external booting by default. For Apple Silicon (M1, M2, M3): Shut down your Mac.

– With strong recommendations.

As a Mac user, you've probably encountered a situation where your computer's internal drive is running low on space, or perhaps you've encountered issues with your current operating system that you just can't seem to resolve. Whatever the reason, you may have considered installing a fresh copy of macOS on an external drive.

Installing macOS on an External SSD/HDD Verdict: Highly recommended for specific use cases, but with critical caveats regarding hardware.