20′ x 24′, basically a double garage space (2 car). Ceiling height?
Before you go hog wild on power amps, consider several things:
How will the space actually be used? Will there be need for people to talk to each other without screaming at the top of their lungs?
With the relatively high sensitivity of the main speakers, is it conceivable that you would really need more than, say, 100 watts per channel, L, R? And at that point, you’d really be needing much more power for the sub(s) in order to keep up with the mains.
If it is a converted garage, will all 4 walls be present (garage door open or closed)? Loss of the 4th wall and its reverberant characteristics, along with the pressurization aspect drastically changes how things work.
A bit of analyzing will help put things in perspective. I will say this, it’s better to go with more power than less. A reserve will get you through those times you might/will need it.