The Deluge jacket is sized to fit under the Basilisk Jacket, the opposite to the pants, which are sized to fit over the Basilisk pants. For that setup, just order the same sizes in the Deluge as you order in the Basilisk.
The Deluge is designed to be a few different things: a backup to the Basilisk for day-long downpours, a packable emergency rain layer to carry dirt biking, or a comfortable jacket to be worn off the bike around camp or town. It's nice to have it fit well on its own, and it's also easy to stop on the trail, take off the Basilisk jacket, and put the Deluge on underneath. The Basilisk is just as waterproof as the Deluge, so it doesn't need to be protected from the rain.
The Deluge pants are sized so that you can put them on over your boots when that day-long downpour starts.