https://drafts.csswg.org/css-page-3/#page-size-prop says:

Computed value: specified value, with <length>s made absolute.

Might it be desirable to also tack on something like ", and with <page-size> converted to absolute <length>s, taking into account any specified portrait or landscape values when choosing which <length> comes first." Maybe this is going too far, but from an author's point of view it seems undesirable to require a table of all paper sizes if you want to convert computed style to an actual size.