I was not aware there was a limit. I ditched FreePBX and Asterisk NOW a long long time ago since I found the GUI impossible. I know what you are talking about when you define how many digits an extension is.
The thing is, I am using Asterisk 1.4.26 directly now and I know I can create 3 digit extensions, 4 digit extensions, and 5 digit extensions. System stuff like call parking and meeting/conference rooms are 3 digits, most employees have a 4 digit extensions, but I also have some branch offices where the managers demanded 5 digits extensions beginning with 50000.
I also just tested 6 digit extensions and 7 digit extensions, even though 7 digit extensions are a bit silly in the U.S as I had the lines at the very beginning of the context for it to work :)
I know it's been awhile, and you probably already had this figured out, but what the heck I just joined and felt like trying to answer some questions.