How large(how much memory is required by this update?) iOS

To be on the safe side, you should have 8-10 GB of free space. The update itself does not take up that much space, but the installation software demands a really significant amount of "overhead" or "temporary" space.

Another way to say the same thing is that I've never seen an installation fail for lack of space when 8 GB free space was available for the update.

If space is tight, you can always use a computer to update, since the computer stores the extra data requirements required during the installation.

It differs. If you are on a older version more. And less if you are on a newer version of iOS. But its around 2-5 gB.

Which update?