There appears to be consensus at least in the current comments on this issue for adding a property to modify scrollbar size, with some additional preference for that to be treated as a maximum size, allowing implementations to show smaller scrollbars if it’s more appropriate.
I have captured the use-cases mentioned here and to me in person on the wiki accordingly:
Regarding the concern about terminology raised by https://github.com/silverwind about using "width" for horizontal scrollbars, vs the suggested alternative "thickness", note that CSS already has a notion of modifying the width of horizontal and vertical "bars" in the 'border-width' and 'outline-width' properties.
In particular note the pre-existing 'border-top-width' and 'border-bottom-width' properties (https://drafts.csswg.org/css-backgrounds-3/#border-width) which specifically apply to horizontal borders. Thus I think it is both ok and desirable to use "width" to refer to the scrollbar size as well, since it is consistent with those existing properties, and matches what web developers will likely already be familiar with in CSS.
I’m going to specify a 'scrollbar-width' property that takes length units that sets the maximum width of any scrollbars on an element when they are shown. 'auto' will be used as the initial value that means just use the platform default scrollbar size.