Reference scale for ArcGIS Online for fixed symbol size

[ShuLee1]

I would like to be able to make my point symbols fixed in size that correlates to the real world - like a box that is set to be 20 square meters.

I do ecological assessments and I would love to be able to make my symbol for a biodiversity assessment survey plot to scale on the field/web maps - it would allow the field staff to be able to plan and monitor more effectively.

ArcGIS Pro allows us to do this via reference scales, but not as a web map on ArcGIS Online and thus not as a Field Map.

The screenshot below shows my survey plot symbol, and how its been set using the reference scale. The centre line is 50m, being able to drop a scale survey plot symbol would really help my field staff with planning placements.