For all length specifications, use proper units of measurement

Bug description

For all user-facing length specifications, please refrain from supporting dimensionless numbers implied to be inches.

Do support numbers only when combined with explicit units of measurement, such as 10in, 17cm, or 150mm.

(Including, but by no means limited to page-width, fig-width, fig-height …)

Please do follow the examples of TeX/LaTeX, pandoc, and pretty much every text processor (MS Word, LibreOffice, Pages, etc.) there is in this respect.

Checklist

• Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
• Please format your issue so it is easier for us to read the bug report.
• Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
• Please document the operating system you're running. If on Linux, please provide the specific distribution.

Start by creating the requested minimal, fully reproducible example in a single .qmd file, covering user-facing length specifications such as page-width, fig-width, and fig-height. Trace how these specifications accept dimensionless values, then verify that the affected cases require explicit units such as in, cm, or mm.