[X4U] How big is a DVD really?

[Stroller]

Answers in metric or imperial dimensions will not be appreciated. ;P

I have always understood writable DVD disks to have a capacity of  
4.7gig, and indeed I have some DVD-Rs here that are marked as such.  
Confusingly, I also have some dual-layer DVDs which are marked as  
8.5gig - I thought the whole distinction between single- and dual- 
layer disks was that the latter have TWICE the capacity.

What I'm REALLY interested in is whether these figures reflect actual  
capacity, or include "overhead" space that will be reserved for the  
file-system.

I back-up customer data on a fairly regular basis and it's getting to  
the point where this is starting to regularly exceed the capacity of  
a single DVD. I'm dumping a lot of it across the network to a Linux- 
based fileserver and what I'd like to do is run a zip archiver (well,  
probably 7-Zip) and have it break archives into appropriate sizes to  
span DVDs. I'll probably include a PDF with extraction instructions  
and maybe Windows- and Mac-based extraction apps.

What size should I make the zip segments?

Stroller.