Go bigger. That's as tall as any of the larger trunks, just narrower. So having a larger trunk wouldn't really change the look of the bike, except for the width.
My trunk isn't the biggest out there, at 45 liters, but the space sure is nice. I normally keep my rain gear, a bungee net, sunscreen, a little first-aid kit, and a couple other things back there. It's really nice to be able to put the helmet on the helmet lock and easily put my jacket in the trunk.
Mine will also hold all that (except for the jacket) and the 12-pack I might decide to pick up on the way home. Or some groceries, or a brief case, books, files, my scanner, or any number of other things I might need.
OTOH, there are some really huge trunks out there, maybe you might find some really are too big. The 'strom is a pretty beefy bike though, not much looks too big on it.
Here's my bike sporting it's Givi E45s. I wouldn't want a trunk any smaller.