Boxers came out of late nineteenth-century Germany, where hunting dogs once used to hold boar and deer were crossed with English Bulldogs. The result is a square, muscular dog of roughly 50 to 80 pounds, with a short, shiny coat in fawn or brindle, a black mask and a blunt, slightly undershot muzzle. Shedding is low to moderate, so a small flat won't fill with fur.
In SF, a Boxer's personality is the main thing to plan around. Many stay puppyish for years, greeting people with wiggles and leaps, so patient reward-based training and supervision around small children matter. Their thin coat and short face leave them sensitive to heat and cold, and the breed carries real heart and cancer risks worth discussing with a veterinarian. Dogs vary, and a Boxer mix may show only some of these traits.