Forms: Checkbox doesn't scale when changing text size

### Impacted plugin

Jetpack

### Quick summary

Originally noticed in https://github.com/Automattic/jetpack/pull/42278#issuecomment-2711244221, but it wasn't introduced in that PR.

If a user tries adding a Checkbox block to a form and changes the text size, the checkbox input itself doesn't scale with the text.

For example, here a checkbox with a small text size:

Here's one with XXL text:

Note that the checkbox is the same size

### Steps to reproduce

1. Add a Contact Form block
2. Add a Checkbox in the form
3. In the Block Inspector, try changing the font size of the checkbox

Expected: The checkbox scales to an appropriate size the text
Actual: The checkbox stays the same size

### Site owner impact

Fewer than 20% of the total website/platform users

### Severity

Minor

### What other impact(s) does this issue have?

_No response_

### If a workaround is available, please outline it here.

_No response_

### Platform (Simple and/or Atomic)

Simple, Atomic, Self-hosted