Full height isn't usually reached until about 18 months - that's when the growth plates close. But by a year or so old, most dogs will be within an inch or so of the final height, they don't keep growing at the rate they did as baby puppies
And most dogs will then continue to fill out until around 2-3 years old.
Not sure that an "average" weight is much use to you - there are plenty of over and undersized individuals around to really skew the stats on that. The boxer standard in many parts of the world does give a range though (and the US standard used to). The dog hasn't changed - so a correct adult
male boxer should be between about 22-25 inches tall to the withers, and weigh around 65-75 lb.
You can view the standards from around the world (height and weight ranges included) at www.boxerworld.com/standards