There are so many theories and different opinons on this subject. Some wait so long because they think their dog will not grow to its full potential, other do it early to avoid agression or because they want a calmer dog.
Really its up to you. Your dog is going to grow no matter when you snip him. Your dog is also only going to get as big or stay as small as his gentic make up allows. Some dogs have huge parents and they end up being small. Some dogs have normal sized parents and end up the size of a small horse. ( I have seen both happen)
As for calm and relaxed puppy..... that doesn't really exist in boxers unless they are tired
Some are more calm than others but they are still boxer puppies.
As for agression, it can help agression a lot. Since non neutered males have a tendancy to have issues with other non neutered males.
I got Charlie neutered at 8 months. My vet told me I could do it earlier but I waited for 8 months because I felt that was a good enough half way point. Charlie is a big boy, he is 80 lbs and lots of muscle. He didn't finish filling out until about 2 1/2 yrs, he is just 3 yrs as of August.
So long post short
it is entirely up to you. Some people do it early, some people do it later. Its all a matter of preference and your opinon.