Quote: They are about 6 mm, little bit of variation, so size 20.
There's often a little difference in size between the stones. Size 20 shell stones are basically anything from 6.0 mm - 6.4 mm. It allows some variation between stones in the manufacturing process. As a normal rule, however, the slate will be one size larger than the shell stones in order to balance the illusion of the slate being smaller on the board.
Quote: How can you differentiate between Mexican and Japanese clam shell? I've tried finding picture comparisons, but have been unsuccessful. I figured they were Mexican clam shell though.
This an get a little tricky, but once you've had a little practice with what to look for it's fairly easy to spot the difference.
Mexican stones are graded on where the stone is cut in the shell (basically the fineness and uniformity of the grain).
Near the base you'll get snow grade which has a fine, fairly uniform grain. About 10% of stones.
In the middle you'll get 30% in moon grade. These also have a fine grain but not as uniform. Some manufacturer's like Mr. Kuroki don't use moon grading anymore which I find a bit of a shame.
Towards to base of the shell you'll get about 60% of your stones, these are the Standard or Practical grade (the wording is interchangeable). There are two types of practical grade that I've seen. One (the kind Mr. Kuroki makes) that is nicer and has a strait grain across and the other where the grain loops in an arc around the shell.
Here are some snow grade Mexican Clam stones...sadly these are old pictures. I'm in China at the moment and can't use my personal collection as an example (and why I'm not selling at the moment on Ebay). Sorry these pictures are rather big...but it will help show the detail 
Hyuga (Japanese) clamshell (Called Suwabute) is not graded the same way as Mexican clamshell because the grain is exceptionally fine and uniform grain throughout. So much so that it puts to shame even Mexican snow grade. Instead Hyuga stones are graded mostly on their color. The uncut shell is quite beautiful and rather colorful. "Standard" grade is called "Flower" grade and will have a warm, orange glow throughout. Moon grade will have less color, but nonetheless still noticeable. Snow grade will be milky white with just a tinge of orange coloring giving the stones a slight warmth.
The grain in Hyuga stones will often have a slight angle to it, and in sizes 28+ you'll often see a curvature/arc in the grain towards the center (more common the larger the stones are...I almost never see it below size 25).
So you can identify Hyuga stones on three points. 1. Grain, 2. Color, 3. Angle/curvature of grain.
You'll never find Hyuga stones larger than size 40...in fact you can count on one hand the number of sets that have ever existed in that size. The shells grow much slower (and not as large) than Mexican shells which is why they are rarer and the price rises exponentially with each size increment. I once held a set of size 40 stones...each one was worth almost two times more than it's weight in gold (at current market value)
I have some hyuga pictures I'll show you, but I can only load three per post, so I'll put them in just a moment.
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. Please treasure them for many years to come 