The length could be 4 months, as all BDs grow at different paces. My female was about 8 inches at 8 weeks, give or take, but that doesn't mean that yours isn't 4 months. The pet store doesn't know because they buy from wholesalers who don't give them any information about what they're sending.
You'll want to do lukewarm soaks a few times a day, as not many actually use the water bowl. Spraying the veggies is a good idea, as well.
It may take time, as most pet store reptiles aren't being cared for like they should, and many actually have parasites or other illnesses, which is why it's generally not a good idea to by from a pet store.
Otherwise, keep offering the veggies daily. You may have to cut back on the insects so that the belly isn't full from them, making more room for the veggies. Try an assortment of things, as well, don't just offer one thing.