Yes, I believe that a 5 CFM fan will work on the newer, larger, 22" WSM.
My 5 CFM(s) works fine on my WSM, my Stump's Smoker GF 222 CM and my Stump's Tailgater. The GF 222 CM is about 1-1/2 to 2 times as large as the 22" WSM, and the Tailgater is NOT really a low-flow smoker.
You can get the larger fans, but in MY opinion, the fan will be off more than it is on, and that could cause larger temperature swings.
I'd try your 5 CFM, and if you aren't happy with it, buy a larger one. You will eventually want a separate blower and pit probe for each of your smokers. (Don't ask how I know this)
BOB