I'd definitely try the 20 gallon; It's a long shot, but she might eat more if she has more room to burn off the energy from the food. Also, if you can get them in your area (tropical roaches are illegal in Canada and I think Florida), maybe try some dubia roaches. They have less chitin and more meat/protein to them than crickets. You could also try Phoenix/Repti worms, they have a high enough calcium content that they don't need to be dusted and they're small and mushy. Her eating problems might be as simple as just her just not liking crickets. What do you offer as greens?
I am honestly really stumped here, and I wish I knew more so I could be of more help. We know she's at least 5 months but with her size there's no way to know her actual age short of asking the breeder, and even then not all breeders keep track of their clutches. If she's 5-6 months that puts her around my Beardie Diesels age, and I've been told Diesel is growing at the average rate. Diesel is nearly 3x her length and 25x her weight though, at 17.5 inches and roughly 175 g. And if she has no symptoms of adeno (apparently babies with adeno rarely make it past 90 days) and is otherwise healthy than her being a "pygmy" is the only other thing I can think of...
Do you have a picture of her you could post?