1. Is every revision saved? Meaning: if I change a
slash into a
backslash in a text of 12 kB is the space needed changing to 24 KB?
And if I change a "then" to a "than" ist it changing to 36 kB than?
Yes, every revision is saved. And yes, this means that the database
can grow quite big quite quickly, but you can set $wgCompressRevisions
to true in your LocalSettings.php to combat this
2. If I delete an article as an administrator, is it
really wiped off the disk?
No. It'll just be marked as deleted, meaning (among other things)
it'll not be accessible to non-admins anymore.