I kept looking at the calibration slides for microscopes and finally decided to invest in one.
It has dots of various sizes and, most useful for me, in the centre is a circle which contains a cross 1 mm across with a scale marked every 1/10 mm and every 1/100 mm.
By taking snapshots of that with each objective lens I could work out the actual size of each shot.
Taking a piece of moss from a hanging basket:
Viewing it under my lowest magnification:
I now know from photographing the calibration slide that the photo is showing a 3.3mm view across with that objective lens.
3 minutes ago