MATLAB: How to characterize these cells by area and diameter

image processingImage Processing Toolboximage segmentation

I have this picture:
The particles of interest are labeled in red. I would like to quantify these labeled images by area and diameter.
For reference, the above image came from this image:
I think there might be a way to take this image and contrast the white to the black and come up with an area and diameter, but I'm not really.
As always,
Thanks for your time,

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