My pelicans aer about the same dimensions of the 20mm boxes.
17.10" x 7.50" x 16.00"
Those boxes along with my trunk give me pretty much all I need for carrying stuff. In the boxes, I carry, first aid kit in a nylon lunch box type carrier. Electrical WalMart Airpump, Spare front inner tube, another smaller type of container that has some TP, firestarting kit, excedring, compass, and alkaseltzer, and my two tire irons. A bank type zip container that holds some loc tite, RVT, Di-Electric grease and other assorted things. All this is in my left box.
On the right side I carry my Army mechanics bag chocked full of tools including a magnetic tray, spare bolts and nuts, zip ties, Charging system tester, large tire pressure guage, hunting knife, field jack, rain suit in stuff bag. The trunk is left empty for whatever else I might pick up on trips or extra clothes. I also have a KLR tailbag that I can mount in front of my trunk that will carry a huge amount of other things if needed.
Pic of cases and trunk:
Side case opened up:
Hopefully this will give you a rough visual of the size of the 20mm boxes might look like on the bike.