I have the same problem to create a mosaic. I get the error of
Product 'sourceProduct2' contains bands of different sizes. Currently it is not possible to use it for mosaicking.
I tried doing the mosaicing operation with the SNAP tutorial, but I cannot achieve to do it.
But when I do the mosaicing operation with one band (for example band 2 from each image) I have not any error and I will get the mosaic.