I think you just answered your own question, David!
The 18mm is a good size for most things. Of course there will be some small items you can't brand, but they're not high-end things anyway.
I played with the brands of all sizes at the seminar and was impressed with all of them. I still need to settle on a symbol before ordering though and I've not got round to it yet. You've spurred me on. Brands look so much better than a pyro'd sig.