It's a Warmoth, 10" to 16".
Stew-Mac recommends switching to different radius blocks as you work your way up the board. Makes sense, but that's a $65 investment, plus the 3M Stik-It sandpaper isn't cheap. But I suppose they would be handy for leveling the frets, too.
I have Larry Robinson's book, and he uses a flat sanding block. But he's a pro...