One of the comments was a question about if I had beads with 2.5mm holes. I had to look at my handy little chart to see where that fit in the mandrel range.
I'm not sure if this is just a United States thing, but why can't everything be consistent? I buy glass rods that are approx 5-7mm in diameter but 13 inches in length.
We give you the sizes of the beads like this....15x17mm but give you the hole size by telling you the mandrel is 5/64".
Widget Supply sent me a business card with my order. It's become dog-eared with use because on the back it has the inch and millimeter equivalent chart.
In making beads you will find these to be some of the common mandrel sizes.
The mandrels are coated with a bead release which makes the hole size bigger, but how much bigger depends on how thick the coat of bead release is.
So to answer the question, I would say that if you are looking for beads with a hole size of 2.5mm you should look for beads made on 3/32" mandrel. Beads made one at a time by hand will always have variances from bead to bead, but that is the beauty of handmade lampwork beads...