macOS can lock the Dock. Apple just never gave you a switch for it.

These are the settings any app for this uses underneath. Set your Dock exactly how you want it first, because these freeze whatever is in place when you run them.

Your commands

Paste this into Terminal and press Return. Applications, then Utilities, then Terminal.

Things worth knowing before you run it

The killall Dock at the end is required. Without it the preference changes, but the Dock keeps running with the old one, so nothing seems to happen.
System Settings will go gray. The matching control stops responding and macOS says the setting has been configured by a profile. The graying out is expected. It is why people panic about it months later.
Nothing records what your Dock looked like before. If you lock at the wrong size, the only way back is dragging the divider by hand.
magnification-immutable does nothing. It appears on several sites as a fifth lock. It has no effect, and we tested it.
It only applies to your user account. Another account on the same Mac has its own unlocked Dock. For a shared Mac you need a configuration profile.

If you find yourself running these more than once, Still Dock, an app that locks your Mac's Dock, exists for exactly that. Same four settings, as switches in the menu bar, and it remembers the Dock you had before you locked it so you can always get back to it.

The commands above stay free either way. Terminal, profile and app compared.

These four are the locks. For every other Dock setting you can change the same way, see every Mac Dock defaults command.

Mac Dock lock command generator

Your commands

Things worth knowing before you run it