I also have and use the 18", but I find that sometimes it's not wide enough, depending on the size of the meat, like brisket.
I still use it and just roll the meat up "diagonally". I think I'd prefer the 24", but I'm not gonna buy more until the roll I have is gone.
18.5", 22.5" OTS
Large BGE (chambersuac's)
The Fiery Furnace (chambersuac's)
Big JT's Smokin' BBQ Competition Team