Returns writer_or_reader's row-group size ("chunk_size", matching parquet_open_writer's
own chunk_size argument name -- Arrow's own WriteTable convenience function uses this same
term for the concept). For a parquet_writer: the caller's own explicit chunk_size, validated
against every declared vector column, or -- if chunk_size was never set -- an estimate
computed from the schema's declared types/col_size. Usable at any point after
parquet_open_writer, including before any column has been written, and it does not change
afterwards: parquet_new_row_group's own nrows is what sizes each streamed row group and
never feeds back into this value, and a writer that only uses parquet_write_column has its
row groups sized at parquet_close_writer from the finished table's real byte size, which a
schema-only estimate here cannot anticipate. So on the write side this is a suggestion for a
chunked-write loop, not a promise about the file's final layout. For a parquet_reader: the
row group's size at row_group (1-based; omitted defaults to the first row group),
reflecting the file's actual, already-written-and-fixed layout -- row groups are not
guaranteed uniform, so it is only ever a suggestion for a chunked-read loop there too.