I will document the entire process, from renovation to duct work to the actually grow! Thanks for the help!I'd stick with the 20" x 36" - you'll be thankful for the extra space when it comes time to add other equipment (humidifiers, circulating fans, filters, etc). A 2' x 2' would be really tight with 2 plants - I'd probably only do that for a single plant.
Make sure you do a grow journal when you start so we can follow along!