I had pre-ordered my first Domino machine, so I was one of many waiting at midnight to accept delivery from my dealer the night they launched. I also pre-ordered the assortment kit, which in those days did not include the yet to be marketed 4mm. Of course that Domino machine as the pins.
Having that assortment kit with all the cutters was very important because I could not be sure which size I would be using for a given project until I gained Domino experience. I did not want to be frustrated needing a size I did not have in stock.
Even before the 4mm came out in the USA I felt the need for a second Domino machine, which has the paddles. That time I waited to by the assortment kit until the 4mm were available.
As the stock of a given size dwindled in the kit I would order a box of that size from my dealer.
To answer the OP's question, I just checked my purchase records from August 2010 (when I opened my new shop) to 1 October 2011. In my shop all 6 of the cabinet makers working for me use Dominoes somewhat more often than I do.
Although my guess would have been that we use 5mm the most, because I use those so much, it turned out we purchased 300 more 6mm than 5mm. We use 10mm the least and about the same number of 4mm as 8mm. Those are all Birch for interior work. Over the past year we have increasingly done more exterior cabinets, for which we use Sipo. I bought an assortment Systainer for the Sipo Dominoes. Since it came filled with Birch, I just removed those and put them used those to restock the other Systainers. I started with the minimum size cartons of each size Sipo. The only Sipo we have needed to re-order are 5mm. So I bought the large box of those.