Dear Lovepreet,
the beam size settings on the BEAM card is overwritten by the source routine.
To specify an extended source, you need to sample the random position. This can be easily done with the sample_flat_distribution() function described at section 5.2.1 in the source_newgen.f file.
Remember, you need to specify the numbers in the source routine as double precision, like 20.0D0.
Cheers,
David