It’s not easy. Not at all.
To complicate things, it depends on where you get the biretta.
I had problems with my first one. I knew (or thought I did) my hat size. A friend brought me a biretta from Europe. He had to convert to metric. It was off just a little. Another time, I thought I knew my metric hat size. That time, there was a difference in how the companies measure (which is typical) and again, wrong size.
It’s like buying a shirt. I wear a size 16 clergy shirt from one company, but from the other company, I need a 1 6 1/2.
One suggestion is to give him a “representative” gift followed by the actual one. Maybe borrow a biretta from someone, or have a picture. Put that in a hat box along with a note saying something like “we need your proper hat size, so this represents your gift”
In the meantime, some notes on buying a biretta:
Birettas made in the U.S. are considerably more expensive; but the quality is far superior (far, far superior).
I found the best prices in Poland. The quality is the same as what I found in Rome.
A collapsible one can be handy sometimes, especially if he travels with it, but the creases sometimes show. A non-collapsing looks better and lasts longer.
Meantime, here’s a few links
(best price I found anywhere)
http://www.barbiconi.it/schedabis.asp?idProdotto=44
(you’re paying for the label)
From what I see, all the stores in Poland use the same manufacturer. Be sure to consider the shipping prices.