The CSS Working Group has published an updated Working Draft of CSS Box Sizing Module Level 4. This module defines the sizing properties (width/height/etc.) and their keywords. It is currently drafted as a delta spec over Level 3 and introduces new features such as aspect-ratio and contain-intrinsic-size (see list of additions to Level 3).
This module is still under heavy revision; changes since the last Working Draft are listed in the Changes section.
Please send feedback by either filing an issue in GitHub (preferable) or sending mail to the (archived) public mailing list www-style@w3.org with the spec code ([css-sizing-4]) and your comment topic in the subject line. (Alternatively, you can email one of the editors and ask them to forward your comment.)