Dude. I've never seen a cat with a tail that long.
That could be an indicator, but a better indicator is down her limbs and body relative to her age.
As for when a cat is "full grown," a lot of veterinarians say that a one-year-old cat is an mature, but in my experience, cats continue to grow after that, especially masculine cats. All my male cats matured in the face (wider face, stronger jaws, etc.) after the one-year mark. They stopped maturing at around two years old.
Answers: I had the same problem with my cat. I thought she only had a freakishly long tail but eventually she grew in to it and is just a big size cat.
I'm certain yours will grow to be a beautiful cat.
Blimey that is a long tail, I also have a drastically long tailed cat, I can hold the tip of it in my fingers when he is walking on the floor without bending down, he is half oriental and weigh 15 pounds, that's not fat by the way, that's all cat, so yeah hes huge, I cant remember what he be like as a kitten, I don't remember ever thinking he was going to be big, he just kept growing, I own his mother as well she only weighs five pounds and is tiny! I know he reach his full size when he was about 18 months old, he is black, similar to yours, and I was glad when he finally stopped getting bigger, I was starting to think I have a panther on my hands!
Wow that is s long tail! But not an indication of how big she will be. Some breeds (Siamese, Oriental types) tend to be long tailed, long slendar body, but may just weigh a solid 7-8 lbs when grown.
I suspect she could have Oriental in the background.
Most cats are more/less grown by something like a year old, but some will continue to fill out contained by body for another year. Maine Coons and Ragdoll cats are not fully grown/developed until 4-5 yrs old.
That's so cool!