Mixed breed isn't a breed but a label for any dog with ancestry from two or more breeds, or from an unknown mix. Most adoptable mixes weigh somewhere between 20 and 70 pounds, though some are much smaller and others much larger. Ears, coat, color and build can come from any branch of the family tree, so two littermates can look nothing alike.
That variety is useful in a city like Jersey City, where homes run from elevator towers to walk-ups. Ancestry only hints at behavior, since research finds breed explains a small share of how dogs act, so look at the dog's size, age, energy and manners on a leash. A DNA test can add interesting detail, but it won't predict daily life.