From time to time there are posts asking for a proper flash for M cameras. I have recently acquired the relatively new Fuji EF-X20 flash, originally made for the X-Pro1, because it actually features a nice manual mode where one can adjust the output power up to 1/64th of GN 20 (m).
Of course it will do TTL with the X-Pro1, but I thought the form factor is actually quite perfect for M cameras, too. It runs on AAAs due to its size - Eneloops are recommended. It is so small that with its pouch, that it fits nicely in a bag just in between the gap the camera and an attached lens form when placed upright, so no space really sacrificed. Downside is the price, I know, so this is a good excuse to later on buy the Fuji host camera in addition, as you've got already the flash and it would to just too expensive to only use it on manual ...