I'm a six-foot tall guy and would say I'm slightly above average in size. I've got a big mouth too (just ask my wife). You'd think that the Radius Scuba brush would be a perfect fit for me. When it comes to fitting in my hand the yes, it's perfect. The design of the handle maximizes grip and precision like no other toothbrush out there. It comes in either right or left hand styles to insure you get the best possible control over the brush action. The bottom portion of the handle feel modeled for my palm, the thumb rest in just the right spot and with ridges for traction, and the opposite side of the thumb rest is dimpled to provide even better grip for your fingers.
And then there is the over-sized brush head. Many people love it, I know I wanted to. I'll admit that it is better than using most standard toothbrushes with its extra coverage and soft bristles. It is also terrific for brushing your tongue. A smaller bristled brush takes quite a few strokes to clean the tongue while Scuba only takes a few swipes. My only complaint is that I found it too large. When brushing it feels uncomfortably bulky. After I finish, it feels like I've missed spots at the very back corners of my mouth. Factor in its high cost and need to replace the whole brush when the bristles wear out, and suddenly I become less enamored with it.
I have found the perfect toothbrush in its slightly more compact sibling, the Radius Source. It costs less than the Scuba, has replaceable heads, and comes with two of them so it's like getting two brushes for the same lower price. When finished, I can dispose of the head and be glad that there is that much less waste going into a landfill somewhere. It's handle is not quite as ergonomic as the Scuba and the head isn't so large, but it is still larger than most toothbrushes and joy to use.
I'm not saying the Radius Scuba isn't a great toothbrush. I doubt it would have such a loyal following if it weren't. It just might not be the perfect fit for everyone, and it is more expensive and more wasteful. In the end, you have to figure out what works best for you, but for me, it isn't the Radius Scuba.