I've found the following to be a simple, successful mix of the previous answers. I've added additional instructions for those unfamiliar with keyboard shortcuts:

1. Open "Preferences: Open Keyboard Shortcuts (JSON)".

2. Add these entries to the JSON:

    {
        "key": "alt+[ArrowLeft]",
        "command": "workbench.action.increaseViewSize"
    },
    {
        "key": "alt+[ArrowRight]",
        "command": "workbench.action.decreaseViewSize"
    },
   

3. Save changes.

4. Click alt/option key (⎇) and left arrow (←).
   Verify sidebar decreases in size.

5. Click alt/option key (⎇) and right arrow (→)
   Verify sidebar increases in size.

If your sidebar is on the right, consider swapping the commands in the shortcuts you add.

Benefits:

1. Allows editing via "Keyboard Shortcuts" interface.
   ^{(cuz it does not require runCommands)}
2. Uses intuitive ← and → keys.
   ^{(from https://stackoverflow.com/a/77051498/11817077)}