Optimal by what criteria?

I'd always split at ~2Gb myself, simply because that's the size that won't cause you tool problems with other potentially helpful tools or media. I normally do that with my VMware virtual machines too, for the same reason (sometimes it's useful to be able to copy them onto some FAT32-formatted USB media, for instance).

There's no other real criterion to worry about. Span sizes only matter for concerns like that, they don't affect performance, so choose a span size (or don't span) based on what you actually do with your images in practice.

One other place aside from FAT32 which is most notable for the 2Gb size limit is ISO9660; for DVDs to be readable from DOS using MSCDEX they have to use ISO9660, not the "native" filesystem for them which is UDF version 2 or later. ISO 9660 filesystems have a 2Gb limit in practice too.