Well, there is no "larger breed" of Boxer; there is one breed and the proper weight is 60-75 lbs. for a male and 50-65 lbs. for a female.
Quite often the smallest puppies end up being the largest adults, but there are no guarantees and a puppy that did not get a good start (one that was "wormy") may be at a disadvantage. There are several questions to ask breeders here: www.boxerworld.com/rescue/questions/
as well as links to other sites with questions to ask.
I would be very careful in MO as it is a large puppymill area.
A listing of Boxerworld supported breeders is here, as well as the various health tests that should be done: www.boxerworld.com/breeders