The error tells you already what you need to do.
In GeoTiff bands with different sizes cannot be written.
You need to resample all bands to a common size first or you tailor your subset that it only contains bands of the same size.
For an example for the resampling you can have a look here:
This question I don’t understand.
You actually don’t need to specify a path within the data folder.
You should not write additional data into it. It will not break the BEAM-DIMAP format but it is not a good habbit.