They vary in size. I had dreams of a nice 18 pound bitch. My bitch won her championship, svelt, at a good 25 pounds. Her niece, our 2nd basenji, was this hard muscled machine and she topped 27 pounds at her best. (Sadly steroids from 4 yrs til death made her usually closer to 32 pounds.) I was at a show about 18 yrs ago and saw some part African basenjis that looked even bigger. But the European and US standards for size are almost identical (1 pound heavier for females in US).
As for the show ring sizes difference, someone who shows internationally can answer. I just know from rescue that most that I see tend to be bigger, not smaller, than standard.
The FCI (European) Standard:
SIZE AND WEIGHT :
Ideal height : dogs 43 cm (17 ins) at withers,
bitches 40 cm (16 ins) at withers.
Ideal weight : dogs 11 kg (24 lbs),
bitches 9 1/2 kg (21 lbs).
Height: 17 inches (male
16 inches (female)
Weight: 24 pounds (male)
22 pounds (female)