I have a medium sized dog (Jake) at 65 lbs and 2 "Jerk Russell Terrorists" at 14.5 and 18lbs. Small dogs can be good in some aspects because if your dog is getting out of line you can always pick it up and move it. With larger dogs that's harder to do. However, we find that our little dogs are generally more aggressive, bark more, and require more entertainment.
Why is this? Part of it is that JRT's were originally bred and trained for hunting - foxes, badgers, rats, and more. Understanding that the word "terrier" comes from the word "terroir", meaning ground, lets you in on their usual problems such as digging. Little dogs have lots of energy. We joke that the JRT's have 2 speeds, on and off - there is no middle ground. If they aren't tearing around the yard or house they are usually napping. One friend has a JRT that we jokingly say turns all houses into a skateboard park for dogs.
The folks at Canine Behavior Center put it best when they say, "a tired dog is a good dog." Usually a minimum of 2 miles of walking each day is good for most pets but in the teen years of dogs you might need a little more. Jake used to need up to 5 or 6 miles of jogging when he was around 2 yrs old to keep him calm. Now that he's 11 he can get exhausted doing 2 miles of walking. I took Jake and all of our JRT's to their classes. They are held in Bellevue and they do a remote class through the Phinney Neighborhood Association. It's really worth it. Usually the class spans about 9 weeks, meeting only 1 night per week for about an hour. You're expected to do training exercises throughout the week to show skill building at each session. It's basically a "train the trainer" type of thing but that's the real catch - it's the owners that need to be willing to train and be consistent with the pet.
Crate training can help to potty train your dog and will provide them a safe place in your home if the dog were to get stressed. It also helps to train for traveling.
How much does a dog cost to own? Well, it depends... is your dog a purebred that has common ailments in its breed? Many purebreds aren't as healthy as mixed pets because of inbreeding. Pets can get the same illnesses as humans, or at least their own version of them, and there is no knowing what may happen over the years of your relationship with a pet when that may or may not occur.
My cat, Morrey, is pretty healthy and he's 19 (I've had him since he was 8). He usually has his annual physical but last year we had a lot of visits to the vet because he decided to take off out of a 2nd story window and ended up with casts on his 2 front legs and with 4 months of treatment. Is he worth it? Yes.
Our previous JRT was found to have a heart problem and we had to give him 2 pills a day for it. There were yearly EKGs and medication costs. In the end, we only had Russell for 2.5 years because he had a heart attack when we weren't home. I still say every dime spent on him, and every rug he ruined during potty training, was worth it because he had the personality of a superstar and we loved him.
Adopting a pet is a huge commitment that comes with a lot of responsibility. Do so with the understanding that this is a life that you are taking into your care.