How to filter the PDB for single-domain proteins with full length 3D-structures solved
If you want to reproduce the model in that paper, you should email the corresponding author asking for the exact set of PDB IDs/chain IDs/residue ranges they used. There is too much flexibility in the criteria as given to get the exact set any other way.

If you want to assemble a similar set for your own work, the RCSB PDB advanced search is very powerful and would let you filter e.g. by max sequence length or for only monomers.

However the PDB doesn't deal with domains (regions that evolve and fold independently) directly. A PDB chain may contain part of a domain, one domain, or multiple domains.

If you want a dataset of protein domains you could use CATH, SCOP or ECOD.

As a side issue, please remember to remove overlap of homologous proteins between the training and test sets. People entering the field often neglect to do that. See for example this comment piece.