Let X, Y,Z be your matrices.
val = 0.5 ; % at this z-value I want radius / diameter
idx = abs(Z-val) < 10^-3 ;
x = X(idx) ; y = Y(idx) ; z = Z(idx) ;
C = [mean(x) mean(y)] ; % Center of circle
dx = C(1)-x ;
dy = C(2)-y ;
R = sqrt(dx.^2) ;
D = 2*mean(R);
Also you can try fitting a circle with the obtained (x,y) using https://in.mathworks.com/matlabcentral/fileexchange/5557-circle-fit
