After getting home, I worried about the rather large head. It turned out not to be an issue in practice, although the website FAQ says many small-jawed adults use the kids' size. The soft bristles really were, too -- for the first time in a decade, my mouth didn't hurt after brushing my teeth -- and the Scuba handle has enough flex that feedback on how much pressure I was putting on my gums was immediate.
The bottom line: I'm so pleased with my Radius that I'm thinking about getting a kids' one for Laurel's next toothbrush in a few months.