One potential solution that worked for us - use the ArcGIS Online Assistant to edit the JSON of the dashboard directly. Find the "indicatorWidget" code block for each indicator you want to edit, and then manually set the fontSize of the topSection, middleSection, bottomSection lines as needed. Make sure you don't have advanced formatting turned on, or I don't think it will work. Maybe start with smaller font sizes (like 20) and work your way up to find the desired size.