Well she has grown maybe, a 1/2 inch? Since I got her, but she's growing really slowly. So recap change her water regularly, feed high protein and high quality foods, and bloodworms/brineshrimp, anything else? And is it true that when they don't grow to full size they can have a shorter life?
When Nix was a baby I use to feed her three times a day, sometimes 4 if I made her meals smaller and one of those 3 or 4 meals a day was always brien shrimp or bloodworms or blackworms depending on the day. Now she is grown up I feed her twice a day and give her bloodworms once a week and live brine shrimp live freely in her tank (until they are all food). Keep the water warm, tank clean, and high protein food and you may be able to grow her out a little more. NLS really REALLY works well too.
For Nix she grew like a little weed, she reached her full size about 3 or 4 months but she may still be growing. I looked at her today and since moving from Texas she seems to have gotten a little longer....and fatter >.> Nix is about 5ish months now give or take. Even my males who was about 5 or 6 months when I got him kept growing until about 7 months of age. Now he seems to have stopped. Kinda tiny for a boy but decent sized overall. I think you still have a lot of hope. If you can get live brine that works like magic. Some fish places sell them as feeders other stores you can buy eggs and raise them yourself which is easy and more cost effective...and kinda fun watching them.
Edit: Also Nix just went through a final color change recently, before we moved to Minnesota and went from almost all blue to almost all pink with bright red fins and blue tips and a purple tint to her body o.o she is crazy colorful. Almost thought she was turning into a boy on me but nope, she still has her egg spot and dinky fins.
You know, that brings up a good point. In the betta community, you read 3" fully grown (for example). Does that mean body or tip to tail tip?
i ask because some things I have read concerning other types of fish vary . The term "length" is different when in reference to show fish, etc.