There is no promises on how a pupppy will turn out, if there was breeders would be in heaven. Couple things.
Male v's Females - that is a personal choice most boxer owners will agree that they are very different but some like the females and some like the males. So if there is a boxer you fell in love with or have been around you that has helped you make the choice to get a boxer you may want to try to get that sex because if you fell in love with a large lazy male that sleeps all day on the dog bed and then plays with the kids at night before sleeping through the night you do not want to purchase a female that will colapse her crate and bounce off your walls running a hundred miles an hour lol..... There are exceptions to any rule. Males tend to be more protective and once they reach full age less hyper. Females Tend to get into a little more trouble they can tend to be too smart for their own good and sometimes can be shy.
8 weeks old is a good to evaluate a puppy to see what they will look like at full grown a puppy at 8 weeks can reflect how the puppy will look confirmation wise.
As for size at full grown you can tell by the size of the parents, and size of the puppy within the litter and size of the puppies paws.
Also females are going to be smaller then males if you are looking for a smaller puppy and maybe get the runt in the litter. Good luck...