I have a requirement where my Action labels should not be shortened usually 'Action labels longer than approximately 12–14 characters are displayed as shortened.Can anyone please help me how to do it?
This is a documented behavior for texts longer than 12-14 characters and that you cannot modify this in its current form. Refer to the excerpt from the Actions Best Practices documentation
Action labels longer than approximately 12–14 characters are shortened when they’re displayed in the Chatter publisher. Keep names short and descriptive.
At this point, it does not look that you will be able to do anything out of the box. And I wouldn't recommend any customization here particularly for this behavior only as it will be an overhead.