So, it arrived today. I chose an eBay vendor of unlock codes, supplied my IMEI and model, and in about an hour, I had an email with my "unlock code" inside. Happily, and very cautiously, I entered the number... all 16 digits of it. I was slow and cautious because most phones offer a limited number of unlock code attempts before perma-locking to their locked carrier. I was saddened by a flash of an error message containing the word "unsuccessful" and a note that I had 7 remaining retries. I spent the next couple of hours exploring other unlock options, looking at other unlock code vendors and seeing if those little SIM card overlays might work with a semi-modern Android phone (inconclusive). Finally, I stumbled on a video of someone successfully entering an unlock code into their Canadian Motorola Dext. I counted as the digits were entered.... 8 digits. There was a space in the middle of the 16 digit code I received. I risked one of my remaining tries on the first 8 digit half... and successfully unlocked the phone. Nice conclusion. I had actually asked the original vendor about the space in the code, but we had a language barrier.
TL;DR: Unlock codes for the Motorola Dext (at least Canadian version) are 8 characters in length.