Congrats on the success feeding. Sometimes leaving them overnight with their prey just does the trick. Try that again on the next feeding (the overnight thing). If it worked, go with it for now. You've been given some excellent advice in this thread.

As far as size, remember you want to compare her biggest girth to the size of her prey, not her mouth or the thickness of her neck. Snakes can open their jaws something like 150 degrees and their bottom jaw is not attached in the middle by bone like ours is nor has the same type of joints that we have at the connection to her bottom and top jaws. In other words she's built perfectly by nature to take prey that is big and whole.