Try giving her an unchopped worm and see if she eats it, might take a few tries as she gets used to it. There is no minimum size, just depends on your ray. My motoros where eating whole worms around 5-6".
I was amazed at the size of food they can eat, at 8 inches mine were taking massive king prawns without problem, these were 5 times a worms thickness. Many people feed black or bloodworms but IMO they are just too tiny when I see mine inhaling whole mussel 1 after the other.
At 6" she should be able to intake a whole worm. My male was 7" and he would take down a 12"-14" long and thick as a pencil nightcrawler but he would only eat 3 max and he's stuffed. Now he's pushing 10" and he can eat 5-6 whole worms in 1 serving. My 12" Female would eat 12-14 in one serving.