Ah, I see. You'll find that the 12mm work better as they fit on the track better. The 16mm are very full on the track. But the 12mm are hard to find in all the various colors.
To get a brown 12mm for example, I had to buy a travel edition Backgammon game and rob one of the brown dice. It's a perfect brown.
For my custom color meeples (lavender, maroon, etc.) I had to spray paint some.