I'll have to get back to you (on December 7th) about the full size of the pack. I can't remember how large it expands to. Most people probably won't want everything though, like with CPS-2.
Thinking about this, I should do one additional add-on pack, which will have all of the older official ROM revisions. So it will be like CPS-2, where the base pack has a single copy (or two in certain cases) of each ROM, using the most recent revision. People who want everything can add-on a pack with every revision of every game. This saves space and makes navigation easier.
So it would be:
1. Base Pack: Most recent revision of every ROM from latest MAME
2. Homebrew/Hacks/Demos/etc Add-on
3. Revisions Add-on: Older revisions of MAME ROMs, for those who want everything
I think almost everyone will go with 1 & 2, and 3 will mainly be for researchers and completionists.