Chihuahuas are perhaps best known for being the smallest breed of dog in the world, but healthy adults vary in weight. There's no surefire way of telling exactly how big a Chihuahua puppy will grow, but you can get a rough idea by looking at a number of factors. If you're sure you want a pooch of a certain size, consider rescuing an adult Chi from a shelter.
Average Adult Size
The average weight of an adult Chihuahua is between 3 and 6 pounds -- that's a wide variance for such a small breed. However, some Chis mature at weights smaller or larger than this when fully grown. The usually measure no taller than 5 inches at the shoulder. By the time they reach 8 months old, they should have reached their full adult height, but they may still fill out a bit are are likely to reach adult weight around 18 months.
Ask the Breeder
If you're getting your Chihuahua puppy from breeders, ask what size they expect him to be when he reaches adulthood. Reputable breeders should have a good knowledge of the pup's bloodlines and should be able to give you a ballpark weight that your pup will grow to. Be cautious of breeders claiming their pups are particularly tiny, as extra-small Chihuahuas are more liable to have health issues. Scrupulous breeders don't encourage the teacup trait.
Look at Parents and Siblings
If you're getting your Chihuahua from breeders, you should be able to see the mother and possibly the father, if they also own him. Beware of breeders who won't let you see at least one parent. Genetics play a big role in a dog's weight, so seeing how big the parents are will give you an idea of your pup's potential adult size. Also look at his size relative to his siblings, if he's smaller than the others he might be set to be a pint-size adult, or vice versa.
Does Size Matter?
Should the size of your Chihuahua really matter? Well, yes and no. Adult Chihuahuas under 3 pounds are more likely to have medical problems and shorter life spans. So, while the tiniest of dogs might be cute, it's not worth the health risks. Buying from a breeder who claims to have teacup Chihuahuas is a bad idea, as you're creating a demand for sickly dogs who won't have the best quality of life -- these are runts of toy poodle runts, essentially. On the other end of the spectrum, Chis over 6 pounds might be disqualified from AKC show classes but are likely to be healthier and more robust. Therefore, unless you plan on showing your dog, him being on the larger side shouldn't matter -- unless he's fat.
When Big Is Too Big
The only time when it's unhealthy for a Chihuahua to be too heavy is when he's obese or overweight. It's fairly easy to overfeed or under-exercise your pup, because of his diminutive size. If your Chi is a healthy weight, you should be able to see and feel both his waist and the outline of his ribs. His belly should also appear tucked when you look at him from the side. If your dog is overweight, take him to see a vet who will advise you how to safely and healthily reduce your pet's weight.