I cut my two fabrics 32 inches, sewed them together, then used 7 inches of a very stiff interfacing for the backing. I did not interface the front "pocket".
The photo above is of the completed roll up from the back. The one below shows a close-up of the roll up pockets.
The first photo is the final product and here it is in my bag (the Christmas bag from Thanksgiving weekend). In playing with it a bit, I found that a looser roll, that was longer, actaully worked better as it filled the purse more completely.
Mr. Security, my husband, contends it makes things easier for the bad guys to just grab and go, However, I think I am going to like it.
You can "roll up" the roll up to different lengths and widths. The only problem I encountered was that once I put the pockets in, I discovered I hadn't allowed enough room for the "width" of the various items. So I ended up one pocket short. I think I am going to like this, as your various items stay in place and are easy to find. I hate pawing through everything looking for something. This should help.