I don't see any reason why the size of an image should make a difference to that code. It may be that your image is an indexed image, in which case that code will not work. To find out if it is an indexed image simply run this code:
>> [X,map] = imread(filename);
and then have a look at the variable map: if this is not empty then your image is indexed, and you will need to convert it to RGB/Grayscale before using that code, perhaps using ind2rgb.
