Ok.
Let me pedant.
Mechanical trail importance in bike stability is still of a disputed importance compared to other factors. That's well evidenced on Internet.
Gyroscopic force applies in equal extents reguardless of wheel size at a given speed if the distrubtion of weight is spread relatively. I may have made that up?

Riding a borrowed Moulton is proof enought to me that they are stable at speed.

Anyway answer to OP is my black 16" mezzo has done 30+ mph on the flat on the A1.
Fast enough.