We've seen a lot of abalone shell smudge bowls, but these are some of the most vibrant and solid ones we've ever seen. They're polished on the inside with a light oil coating, and left natural on the back, and they're between 5" and 6" on average.
We've seen those sellers that say large, but you get a little pale looking thing when you order it. These are properly large - the main photo shows a 4" smudge stick in it, for comparison.
You can extend the life of your shells if you add some sand in before you burn stuff, but of course it can be burnt right on the shell - just expect some scorch marks as you use it.