OpenCV reference manual (2.4.x) states that the constructor that initializes MSER requires the following parameters:
delta, min_area, max_area, max_variation, min_d
I found the answer to my question thanks to this link.
MSERs are obtained by varying the intensity threshold T from 0 to 255. Delta decides the least count of that variation. So, size{i} means the size or area of region with intensity value i in a grayscale image.
Will get back with an explanation of MaxVariation and MinDiversity soon.