She is a big puppy indeed! The maximum height for boxer bitches per the breed standard is 59 cm (23 inches, for those who think in imperial measures), and that refers to adults. Your little girl is over that and still plenty of growing time left!
As a rough average, I think that most boxers are probably within about 3 inches of their final height by the seven month mark. That is an average though, and they do grow at quite vastly differing rates. Some dogs still haven't reached final height at 12 months of age, whilst some others are mostly done by about 9 months. It is quite possible that your pup just happened to be one of those that sprouts upward very quickly and may then be much closer to final height than her 7 months would suggest (i.e. she might not yet grow 3 further inches). And if not - then you're going to have a giant amongst boxer bitches on your hands