journaled FS and and WAL

Hi,
two question related to the WAL.

1) I read in the doc that journaled FS is not important as WAL is
journaling itself. But who garantees that the WAL is written correctly?
I know that it's sequential and a partial update of WAL can be discarded
after a restart. But am I sure that without a journaled FS, if there is
a crash during the WAL update, nothing already updated in the WAL before
my commit can get corrupted?

2) Let's suppose that I have one database, one table of 100000 rows,
each 256 bytes. Now, in a single SQL commit, I update row 10, row 30000
and row 80000. How much should I expect the WAL increase by? (supposing
no WAL segments will be deleted). I could guess 8192x3 but I'm not sure

Regards
Pupillo