These guys get about 2-3 inches but this guy is not quite at 1:1. I will be back again this summer and will try and get a head shot (and tone down the flash).
Use different materials and run some tests. I usually do this then go with the best looking diffuser material. I ended up buying a white piece of plastic sheeting for .98 cents and made my own diffuser for my flash head. You can see the diffuser in use in my high key flower image. Let me know if you need more info.
Thanks Robert and Mike. Robert, how do you attach the plastic to the flash heads? Did you cut into a box shape or just a "U" shape. I have seen them available commercially but can not see myself spending the money on something I know I will loose when wondering aorund.