Hi arun,
Here's some code to do what you asked, but with everything in units of the graph/plot (not mixing mm / pixel coordinates):
figure;
x=3;
y=5;
plot(x,y,'*')
boxHalfWidth = 1; %
boxXStart = x-boxHalfWidth;
boxYStart = y-boxHalfWidth;
boxWidth = 2 * boxHalfWidth;
boxHeight = 2 * boxHalfWidth;
rectangle('Position',[boxXStart boxYStart boxWidth boxHeight])
axis([0 10 0 10])
axis square
You can change the boxHalfWidth variable for different box sizes.
