Q:

optimum slice size for backing up data to DVD

How big should a slice be for a DVD, assuming that the file system restricts the size?
Referencing:
http://ubuntu.5.n6.nabble.com/DVD-file-size-td4572772.html
should slices be 1GB or so?  What's an efficient slice size?

A:

According to the Wikipedia article,

It is, however, possible to circumvent this limitation by using the multi-extent (fragmentation) feature of ISO 9660 Level 3 and create ISO 9660 filesystems and single files up to 8 TB. With this, files larger than 4 GB can be split up into multiple extents (sequential series of sectors), each not exceeding the 4 GB limit. For example, the free software such as InfraRecorder and mkisofs as well as Roxio Toast are able to create ISO 9660 filesystems that use multi-extent files to store files larger than 4 GB on appropriate media such as recordable DVDs.

So a slice can be as large as a DVD (about 4.3 GB binary for a single-layer disc).
Limited file size
If, for whatever reason, you need to limit the file size to no more than max_file_size and the disk holds disk_size, make your slice size the largest disk_size / s less than max_file_size, where s is an integer. That way, you can fit s slices on the disk exactly (ignoring round-off error), wasting no space. For disk_size ≈ 4.3 GB and max_file_size = 1 GB, you could then make each slice 880 MB, fitting 5 slices per disc.

