I didn't know the clamps came in different sizes? I think they are all the same.
It would be helpful if you could post a picture of the Stone clamp, so that we can see what it looks like.
I personally don't recommend using a clamp tattoo; a pen would be much more reasonable, and if you aren't getting into breeding/showing, I don't see the reason to spend a load of money on something you might use just a few times. Instead, you could go to a show, or contact a local breeder and ask them to tattoo your rabbit for you. They often will just charge $1, which is a heck of lot cheaper than buying a whole tattoo kit and the needed supplies.
Yes they have letters in 1/4, 3/8, 5/16 etc......I found that 1/4 is what is in the rabbits I have now. Can human tattoo ink be used? I was reading a site that said she didnt recommend using india ink and only use commercial tattoo ink. There was no like to contact her. So I was wondering if she meant human tattoo ink. It would seem to me that it would stay better and more bold then the rabbit ink. My handwriting is not good enough for a pen style tattoo.... I prefer the look of the clamp better. And for 4h and show rabbits I would like to tattoo them myself when someone buys them. Also with as many rabbits as I will be housing it will make it easier to keep track of who is who. I have decided on The Grand Champion from bunnyrabbit.com instead of the Stone brand.