(Roughly speaking, to get started, this would mean compiling with --with-wal-segsize 16, 32, 64, 128, 256, run make check-world both sequentially and in parallel, and take note of a) passing, b) run time, c) disk space.)
The attached patch updates a pg_upgrade test which fails for higher segment values: The output of SELECT restart_lsn FROM pg_replication_slots WHERE slot_name = 'slot1'}.
The following are the results of the installcheck-world execution.
For every test, except for connect/test5 in src/interfaces/ecpg, all else passed.
We can see that smaller chunks take lesser time for the same amount of WAL (128 and 256, 512 and 1024). But these tests are not large enough to conclude.