I transferred my burms to rabbits at about 6 feet. One took the swithch with absolutley no problems. The other two however still won't take a rabbit on there own so I feed them a rat and then as they are swallowing down the last bit of the rat and just the tail is sticking out I place the head of the rabbit in its mouth. This almost always works. The gal that has been eating rabbits regularly has gone through some outstanding growth. In about a month and a half- two months she grew 15 inches and gained 8.5 pounds. She is now just under 10 feet and weighs close to 50 pounds.
So to answer your question I would definitely bump up her food size to atleast collosal rats if not small rabbits. Sorry about the ramble, Todd