Hi JD,
You can use the below query to get the WAL file size location.
PostgreSQL-9.x
select pg_size_pretty(sum(((pg_stat_file('pg_xlog/'||fname)).size))) as total_size from pg_ls_dir('pg_xlog') as t(fname);
PostgreSQL-10.x
select pg_size_pretty(sum(((pg_stat_file('pg_wal/'||fname)).size))) as total_size from pg_ls_dir('pg_wal') as t(fname);
Hope this helps.