storybook/docs/_snippets/storybook-preview-change-viewports.md

export default {
  parameters: {
    viewport: {
      viewports: newViewports, // newViewports would be an ViewportMap. (see below for examples)
      defaultViewport: 'someDefault',
    },
  },
};

// Replace your-framework with the framework you are using (e.g., react, vue3)
import { Preview } from '@storybook/your-framework';

const preview: Preview = {
  parameters: {
    viewport: {
      viewports: newViewports, // newViewports would be an ViewportMap. (see below for examples)
      defaultViewport: 'someDefault',
    },
  },
};

export default preview;