Depends on what you mean by "small"... I say this because I noticed that you didn't necessarily specify, "lightweight". If you're already shooting mega-formats, perhaps an old Graflex Stroboflash or Norman 200B flash (what are they? 200ws?) would do the trick? They're relatively small. Or how about something like the Profoto B600? Not as lightweight as a shoe-mount flash, but all the power (light) you might need, and certainly portable (the head can be easily handheld)..
Just an idea ..