It all depends on the dog, but another important part is what kind of personality and tempermant your small dog has and what kind of behaviors displayed by big dogs he would feel insecure around. You should first base any general decision you make about the kind of dog(s) you're thinking about getting next on your Chihuahua and how he reacts to other dogs, especially those that are bigger than him, and pick a dog who will be tolerant of that. Many small dogs feel fearful and insecure about bigger dogs and will snap or yelp and cry, especially big dogs who tower over them and wag their tails in excitement and sniff them all over or dogs who come at them playfully. If you do introductions with a dog you like or decide to do a trial with a dog you like make sure you stay calm, your Chihuahua might sense you feeling insecure and freak out because of it. Where were you thinking of looking for another dog? I wouldn't even trust my dogs with a small dog unsupervised. They would get far too excited and want to play with it like a toy not to mention Karma's tail would smack it in the face.