Gixxer - I'd say that sounds fairly normal, though she's likely to end up on the larger side of the boxer standard or perhaps exceeding it slightly. I had a girl who reached 22" at seven months, and who ended up a touch under 24" as an adult. That's a little larger than she was "supposed" to be - the boxer standard calls for females to be between 21-23" tall - but not so large as to be mistaken for a bullmastiff or anything like that LOL
If I recall correctly, my girl weighed a little less than yours (around 20 kg at that age) but she really was the "skinny" type as a puppy.