I can definately attest to Denny Manufacturing. I worked for the Picture People for 6 years and that is where they got theirs. I measured one a number of years ago it came out to be 18' wide and 24 long. I'm not sure they make muslins in that size anymore though. It was perfect for larger groups. I shot a family of 34 on it (ages ranges from newborn on up). The only problem is the cost. Denny is probably one of the more expensive out there. But if large is what you want, they might be one of the few that carry what you need. I got my roller system through Backdrop outlets and it is great. The service there is amazing and the prices are the best part. You may want to check them out first to see if they have a size to fit your needs. And as a last chance effort to help ya- I read somewhere on the web that someplace like JoAnn Fabrics or a craft supply place online has huge sized canvas/denim that you can order in just about any size you'd like. Then you can just dye it on your own.
Hope this helps!!!