Thank you for your reply!
You have given me some things to think about.
I agree with you on going larger, not taxing the unit at 100%, and to consider cfm loss due to friction. Regarding the register... I was going to place this directly overhead where I smoke, but I am concerned about the potential noise. As you said, I can play with the duct size and register type.
I do not plan on installing a dampener. The garage is not heated so no worries about heat escaping to the outside.
Also, that is a very interesting idea using a smoke machine ... or I will invite everyone from my lounge over for testing as well!!