Buzzer 20.
Everywhere else in the world the buzzers are really small. I've seen UK buzzers that wouldn't look out of place on a size 10, or even size 8, hook. It's handy to have some small ones however, in fact even in the UK a size 20 buzzer can be all the fish really want.
The "trick" to small flies is to offset the hook.
Tying instructions:
size 20 hook - this one's a fancy shape - olive thead body, olive mole thorax.