The numel(M) function returns the number of elements in the array, M, or the number of elements in the subreference M[i,j].

numel does not perform bounds checking of the subreference index to assure that the index is valid.  Therefore, numel(M,5,5) will still return 1, the size of a subreference of the form M[5,5], even for a 2 x 2 array, M.  

The special string, ":" can be used to denote the range of an entire dimension, for example, numel(M,":",1).  Or, ":" by itself as in numel(M,":") can be used to denote the whole array, which is equivalent to numel(M).