Who are you guys?
14 pound rat terrier
It is true that a rat terrier comes in difference sizes. And there are some ratties (Deckers) that can be up to 45 pounds. That said.
1. My rat terrier is a standard rat terrier. He weighs 17 pounds.
2. It's really a combination of the height and weight (so it's about proportion). For instance, if your dog (at the whithers) is 18 inches than perhaps 35 pounds is appropriate. You need to measure your dog where the shoulders meet the neck (ie:before the neck starts to rise upward). To give you an idea, my dog measures 14.5 inches at the whithers.
3. Based on appearance, a rat terrier should have a massive "tuck-in." When folks who've never seen a rattie before ask me what once looks like, I tell them "imagine a whippet in a trash compactor." Specifically, different ears and face, smaller legs but that same big chest and no belly. That's also a way of telling a rattie from a JRT. A rattie will have a bigger chest and then almost no belly whatsoever while a JRT has a smaller chest and the belly is only a little smaller than the chest. When I say a massive tuck-in, the chest of your dog should be almost twice the size of her belly (if you put a tape measure around the chest and one around the belly).
You should be able to clearly see your dog's ribs when she lays on the ground. You should be able to run your hand over her side and count the ribs without pressing. There are also some websites online that tell you about pinch tests near the spin if you use google.
Without seeing your rat terrier and measuring her, I can't say for sure. But just based on a guess, I'd say that your girl is probably significantly overweight.
If that is indeed the case, here are some tips:
1. Start walking with her. My guy (on days that he isn't doing agility trials or there isn't a downpour outside) gets an absolute minimum of 60 minutes of vigorous walking a day and usually it's more like 90-100 minutes. He gets 15-20 minutes per day of fetching a ball (and this is at a sprint for him). I take him running 2-3 times a week with me. He gets 10-20 minutes of agility practice every day (unless he has a class in which case it's an hour). We also spend some time doing tricks (some of which are vigorous like having him jump into my arms). He also chases squirrels and chipmunks in his spare time (that's his second job). Okay, your rattie doesn't have to be that active. But start with something.
Next, portion control of food. My rat terrier is supposed to get 1 cup of dry food a day (I don't feed him wet food--it goofs up the portion control). He gets 1/4 a cup in the morning. All the rest of the food I set aside in a bowl and he earns it one piece of kibble at a time doing things. He sits on command--good boy and he gets a piece of kibble. Every time he looks up at me during the walk--I toss him kibble. When we do trick during the day--more kibble. Ratties love to please, love to do tricks and if you feed your dog her full portion and then add in EXTRA food, that will pack on the pounds.
Next, choice of food. Do NOT switch to a low-fat or weight maintenance food. Those are for dogs that are aging and not active--older dogs. They have low protein in them and low fat. But for a healthy adult dog, fat in the food is good--it builds muscle and quick energy. While the thing in dog food that makes dogs fat and obese the quickest are carbohydrates, especially wheat and corn. So depending upon what type of food you have you might want to switch: get something with at a minimum of 24% protein (and 28% wouldn't be bad), a higher fat percentage (ie: no "low-fat" or weight maintenance food), no wheat, no corn, ideally very few other grains.
Don't assess weight by the stuff you see in books or online. When it comes to rat terriers, there are a lot of other breeds mixed into various lines to create what is eventually a rat terrier. If you have some Decker in your rat line, those dogs will run big (Deckers were bred to help hunt bears). Other ratties look like slightly bigger cousins of toy fox terriers. My guy is 7 pounds heavier than his dad and 11 pounds heavier than his mom--the breeder was astounded he turned out so "big." So with this breed, it's not what you see on a piece of paper. Instead, go by the weight versus the height. If your dog is 15-16 inches at the whithers than 35 pounds is way too much. If your dog is 18-19 inches at the whithers than 35 pounds could be on the thin side.