As per my understanding it is required to detect the circles from the image/video and measure the diameter of the detected circle.
You can use the following code to detect the circle from image attached and calculate the diameter.
%Load the image
a = imread('image.jpeg');
imshow(a);
%Detect the Center and radius
[centers,radii] = imfindcircles(a,[20 25],'ObjectPolarity','dark', 'Sensitivity',0.97);
%Calculate the diameter from radius
diameter = radii*2;
%Visualize circles
viscircles(centers, radii,'Color','r');
In the ”imfindcircles()” and it may also need higher 'Sensitivity' level for better detection of circles. In the current code it is 0.97 which detects all the circles in the image.
The below attached is the image that shows the detected circles from the image:
