The size is appropriate for forms and slightly less wide than`.page`
On iOS, `.form` sizing enforces a platform-defined floor for the vertical
and horizontal dimensions. On macOS, no floor is enforced, however a maximum
proposed height is derived from the presenter height. To achieve
presentations outside of these bounds, see ``PresentationSizing.fitted``
or implement your own custom ``PresentationSizing``.
- Seealso: ``PresentationSizing/form``