In general, IJs are smaller than jungles, but that's just a generalization, and there are some really big IJs and some really small jungles. They're really calm and easy to work with, and I haven't had any trouble swithing all of mine to rats.
If you're looking for something smaller but bigger than a corn, you might also look at spotted pythons or chindrens pythons. Again, there are big ones, but in general they stay under 5 feet.
All of them are very easy to keep and generally very handlable. Keep in mind that carpets are long and thin, so a 7 foot carpet is really less work to handle than even a 5 foot ball, since a good foot or so of that is tail.
Buy a CBB animal from a reputable breeder for the best odds of getting a nice-looking, calm, rat-eating carpet.
ryanm