You want the meal to leave a sizable lump in the snake, but not so much that the snake looks uncomfortably distended. At 7 feet, I'd say two xl-jumbo rats should be fine for her. I'm not entirely sure of the sizing on the rats you're buying, but they should be slightly fatter than the snake is.
This guy's first picture is of a carpet after it's had an appropriate meal:
More pictures of a Jungle Carpet Python | Peter Jolles
Keep in mind too, that carpet pythons are naturally slender snakes. They should not be as thick as a ball python, and a lot of people (vets included) who are not familiar with them might think a carpet is too thin when it is just right in fact. You should not be able to see any distinct spinal vertebrae or ribs, but there should be a subtle indication of the spine's location (basically a small dent in the skin there). If you see a pronounced spinal ridge or can clearly make out ribs, the snake is too thin. If the snake has thick jowls or pudgy cheeks it's overweight.
Here are a few pictures of healthy carpet pythons (you can see what I mean by the spinal outline):
http://www.allexoticpet.net/wp-conte...pet-Python.jpg
This carpet is too fat....note the puffy looking cheeks:
Darwin Carpet Python (newby)
Hope this all helps!