It seems there a way to force Drill to store CTAS generated parquet file as a single block when using HDFS. Java HDFS API allows to do that, files could be created with the Parquet block-size set in a session or system config.
Since it is ideal to have single parquet file per hdfs block.
Here is the HDFS API that allow to do that:
Drill uses the hadoop ParquetFileWriter (https://github.com/Parquet/parquet-mr/blob/master/parquet-hadoop/src/main/java/parquet/hadoop/ParquetFileWriter.java).
This is where the file creation occurs so it might be tricky.
However, ParquetRecordWriter.java (https://github.com/apache/drill/blob/master/exec/java-exec/src/main/java/org/apache/drill/exec/store/parquet/ParquetRecordWriter.java) in Drill creates the ParquetFileWriter with an hadoop configuration object.
something to explore: Could the block size be set as a property within the Configuration object before passing it to ParquetFileWriter constructor?