Have you not seen them in person then. When they say something is small, its well - small. When they call these Micro, they are to say, erm....micro. As in minute. you could fit 3 or 4 of them easily on your finger nail alone and still have room for more. So £3 a pop for one, in a 30L BiOrb, i'd say your going to struggle to see them still.
Very interesting little crabs as i've said, but certainly more than 3, else your going to be looking for a needle in a haystack every time you want to view one!