Postgres COPY with multiple small chunks
You can use a WHERE condition:
COPY (SELECT * FROM tab WHERE id % 3 = 0) TO '/dir/file1.csv' (FORMAT 'csv');
COPY (SELECT * FROM tab WHERE id % 3 = 1) TO '/dir/file2.csv' (FORMAT 'csv');
COPY (SELECT * FROM tab WHERE id % 3 = 2) TO '/dir/file3.csv' (FORMAT 'csv');

Here id stands for any numerical column. You can also use other data types and split up the data in other ways with an appropriate WHERE condition.