Comment # 1 on bug 1124282 from Peter S�tterlin

The suggestion will be by far too little for this kind of updates.

You usually do a snapshot before installing, so everything that gets installed
will be there twice, the old one in the snapshot, the new one in the actual
filesystem.  

So you would need to check for the total installed size of the packages to be
updated, and make sure *that* space is available.  That should in all cases be
larger than the (compressed) download.