Imaging the following scenario:
How our design tools are helping you to understand the Limits of a possible customization?
You tried to shrink the width by 10 centimeters and the blue quadrat shows you what you tried to set and how it looks. This should give you the feeling to do something more in case you need exactly that width.
After I removed some columns it looks better for me. The “Headline” of that WebPart is still a bit wider than my needed width for the WebPart. These limitations are by design and the editing tools will give you hints when you are going beyond the limits. Every HTML designer playing with CSS and more are able to tell you a thing about it.