I'm not sure whether you want only the rows to fill the horizontal space, or actually magnify the visual. If it's the former, I think all you need to do is
not set the row control's width. ZO_ScrollList sets TOPLEFT and TOPRIGHT anchors on rows, so it fills the horizontal space for you, unless you prevent that with fixed width.
Example from
https://www.esoui.com/downloads/info...rTorchbug.html:
Here a template with a ZO_ScrollList anchored to the TOPLEFT and BOTTOMRIGHT of the parent control (i.e. the viewport automatically resizes with the parent control):
https://github.com/merlight/eso-merT...es.xml#L56-L63
Here a template for list row (notice missing x= in Dimensions):
https://github.com/merlight/eso-merT...s.xml#L89-L103
It's been years since I wrote this, but code search reveals no SetAnchor calls applied to rows, which I believe would be the only way to resize them after ZO_ScrollList's normal anchor setting.