The bag easily folds in half and has several button like loops along the side to help prevent it from slipping as you roll it up. I had no problem rolling it up or using the attached cords to secure it. The cord is adjustable to make it possible to roll it very tightly or more loosely. There is a label on the bag that shows you how to roll it up which was very helpful the first two times.
I was trying to figure out the same thing. The measurements on the bag covers available in the market are not helpful in figuring it out.
I'm thinking this one should be right http://www.amazon.com/gp/product/B00AJVP0CS/ref=ox_sc_sfl_title_1?ie=UTF8&psc=1&smid=A3GH7O9G66DJQL. It is by Coleman.
The sleeping bag has a built in string system to hold itself when rolled up. Directions are attached to the bag on how this works, it is simple to do. A separate bag would be unnecessary and another item to keep track of.