Let's say that your image is I. Then using the follow code you will get a 128x128 ROI on your image. You can move and drag the ROI using your mouse. Once it is located at the place you want just double click inside it.
You can also change the values of S to move the ROI.
S = [1 1 128 128];
I = imread('cameraman.tif');
h = imrect(gca, S);
fcn = makeConstrainToRectFcn('imrect',get(gca,'XLim'),get(gca,'YLim'))
position = wait(h);
I2 = imcrop(I,position);
Hope that helps you.