Fretboard Radius Sanding Blocks

Nomad

Another noob question...

What's a good length for a fretboard radius sanding block? I've been looking around at what's on offer, and I'm leaning towards the ones from Crimson, partly because they have the same radius on both sides (so coarse and finer paper can be used, although I don't know how useful that really is). I was also thinking of getting the 400mm one on the basis that I can cut it into, say a 250 and 150 (or a 300 and 100, etc) so that I can do longer strokes for general shaping, and use the shorter one for more localised fettling if I'm a bit cack-handed with the longer one. Works out at a pretty decent price at £25 for the 400mm.

At the moment, I'm only really interested in a 12" radius, and not sure what I'd change to if I wanted something different, I see limited value in spending about the same on, say, a 10/12" that's half the length, only to find that I want to go flatter.