Also noteworthy (but logical): if you plan on shooting with wide angle lenses, like Richard did, you really need the larger filter (and holder) sizes to cover the angles; otherwise you'll have the edges of the filter/holder showing in the corners of your image.
And larger means more expensive...

We do not see things as they are. ǝɹɐ ǝʍ sɐ sƃuıɥʇ ǝǝs ǝʍ