Get a binary image by thresholding. Then fill holes, label, and call regionprops().
binaryImage = grayImage > 10;
binaryImage = imfill(binaryImage, 'holes');
labeledImage = bwlabel(binaryImage);
measurements = regionprops(labeledImage, 'EquivDiameter');
theRadius = measurements.EquivDiameter / 2;
By the way, instead of the very verbose and overly complicated "bidimensional (N x N) matrix of pixels", most people would simply just say "2D image".