Re: increasing the default WAL segment size

At this point, I suggest splitting this patch up into several
potentially less controversial pieces.

One big piece is that we currently don't support segment sizes larger
than 64 GB, for various internal arithmetic reasons.  Your patch appears
to address that.  So I suggest isolating that.  Assuming it works
correctly, I think there would be no great concern about it.

The next piece would be making the various tools aware of varying
segment sizes without having to rely on a built-in value.

The third piece would then be the rest that allows you to set the size
at initdb

If we take these in order, we would make it easier to test various sizes
and see if there are any more unforeseen issues when changing sizes.  It
would also make it easier to do performance testing so we can address
the original question of what the default size should be.

One concern I have is that your patch does not contain any tests.  There
should probably be lots of tests.