Just testing the difference in voltage by changing the magnet size on a four cell. Surprisingly, it appears that the voltage doesn't increase nearly as much as expected especially with the vast difference in the size.

3mm x 1mm

3/8" x 1/16"

1" x 1/4" (N52)

I'm sure there are a large range of variables that come into play but on a first try this is interesting. One thing I did notice from the smaller vs larger was the larger did provide a brighter lighting of the LED.