Ad Block: More Responsive Options

### Impacted plugin

Jetpack

### What

Suggesting to make the Ad block more responsive, automatically or manually

### How

Currently, the Ad Block offers:
- 4 fixed sizes:
Rectangle (300 x 250px)
Leaderboard (728 x 90 px)
Mobile leaderboard (320 x 50 px)
Wide skyscraper (160 x 600px)
- The option to hide it on mobile devices

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The 4 sizes are not responsive. Meaning, picking the "Leaderboard" size affects the mobile layout and adds unexpected white space on the side:

The solution offered currently is to either hide it or change its size to Rectangle.
It's not ideal, a user may want to still show the Ad in mobile, just without breaking the site or changing the Desktop view.

---

Proposal:

- Add a select box to the block where one can select the desired size on mobile (default to Rectangle).
- Or, add the option to hide on desktop. Meaning, a user will have to add two blocks, one to show on desktop and one to show on mobile.

No files, tests, or entry points are named. Start by locating the Jetpack Ad block implementation and reviewing how its four fixed sizes and mobile-hiding option are handled; done means supporting a mobile-specific responsive choice or desktop/mobile visibility without unwanted mobile whitespace or changing the desktop view.

Written by the indexing model from the issue text.