What size room for the Magnepan 20.1?

Do not kid yourself, 19x16, your 20.1 will just love those dimensions.
4 feet away from the 19 ft wall and 6 ft between panels will generate a wide sweet spot and to mention dept and height. You may not need any room treatment and your tweeters can be on the outside.
Your system will sound fantastic, you will hear the drummer changing direction of his sticks or when he taps the left or right cymbals.
Enjoy.

The real trick with Mag's is they need to be at least 3 foot off the back wall, this builds the soundstage and is one of the biggest requirements as I have found.

You will be sitting a little near field but the room is big enough... try flipping tweeter on the inside instead of tweater on the outside to reduce sidewall interactions. Also you can try both speakers on the long and on the short wall to find out what works best for you.

450watts into 4ohms sounds good, but as with all magnepans they love as much power as you can throw at them.

Have fun!
Chris