There are 4 plates to cover A-Z (and a few extra letters to fill in space), plus 3 extra plates to give duplicates of the more commonly used letters. In total, the 7 plates should give the ability to build practically every word.

Notes:
* The font used here is "Anton", from the Google Web Font directory.
* Letter frequency built from Wikipedia's "Letter frequency" page.
* Letters are ~40mm tall.
* Each letter fits 2x BuckyCubes (RIP) which are are ~4mm cubes.

If you want to modify the OpenSCAD script to suit your needs:
* Download "More Fontz!" (http://www.thingiverse.com/thing:13677)
* Customize the magnet locations to fit via trial and error.
* You'll probably have to make your own plate configurations.