A few of the distributors who sell in the Buy / Sell section of this website carry them from time to time. You just have to keep an eye out for them to pop up.
42-43 inches. It's the only big one I could find with a measurement by it.
Now I'm not saying a 4' armatus is normal by any stretch of the imagination, but it is possible. There are some absolute giants that easily rival or surpass that one in size both on Google and in the "Armatus ID Thread" as well, that people caught while fishing.