Short answer: There is no setting for that. The list of choices is hard-coded into a Workspace Component and there is no Component Attribute property exposed to override it. You could try making a feature request on HI to have a Component Attribute added for this.
Longer answer: The list component you are seeing in Agent Workspace is a part of the UX Custom Content Extension called List Module, which you can find in your instance with this URL:
https://<your-instance>.service-now.com/nav_to.do?uri=%2Fsys_ux_custom_content_root_elem.do%3Fsys_id%3D2bec17875300330030c3ddeeff7b12ee
You can see and modify the available Component Attributes on this UX Custom Content Extension page. None of the available attributes allow you to override the row limit choices.
You will see there that it uses a component called sn-workspace-module-list. However, that component uses the now-record-list component, which itself uses the now-pagination component, which is rendering the selection dropdown using the now-dropdown component.
Looking at the javascript source for these, it becomes clear the choices are defined in the sn-pagination component. You can find the minified source for that at this URL in your instance:
https://<your-instance>.service-now.com/scripts/@servicenow/now-pagination/now-pagination.min.js
I used the atom-beautify addon in Atom code editor to make the script more readable and found the following in the middle of a function that was setting a bunch of properties:
... t.LIMITS = [10, 20, 50, 100] ...
So as you can see, the choices are hard-coded into the component and there is no property exposed to allow overriding. You would need the ability to build and deploy a modified version of the component in order to change this. ServiceNow has made development files for a limited number of components available so far, but now-pagination does not appear to be one of these.
