Mastiff These dogs have big hearts, not big energy. So long as your space is large enough for a Mastiff to comfortably move around and you’re up for a daily stroll, a Mastiff will be good in your home. Be prepared for a big head in your lap each time you sit down. Gentle giants indeed. Depending on your size preferences, you can choose from a few sub-breeds, such as the 100-pound Neapolitan Mastiff all the way up to the 200-pound (or larger) English Mastiff, like the one pictured at left.