Gutenberg block preview width

Hi there.

I’m trying to tweak the preview of a Gutenberg block to make it wider.

From the docs

It’s also possible to define the width of the preview container in pixels via viewportWidth

https://developer.ww.wp.xz.cn/block-editor/developers/block-api/block-registration/

This is the code

registerBlockType('my-plugin/dummy-block', {
	...
	attributes: {
		...
	},
	example: {
		attributes: {
			...
		},
		viewportWidth: 1300,
	},
	...
});

This is not working unfortunately.
I’m running WordPress 5.6

Any ideas?