You can find a board that I designed here:
http://www.briannwebb.us/. I actually have a newer design that uses some smaller components, and has all the components on the top of the board. I think I designed the first versions of this board when I was still on the OP project.
The board is 24x24 mm, and has 20mm hole spacing, which is the same size that's used by some of the inexpensive nano quads. I also designed a 15mm square "IMU" board that has the sensors and the 48 pin QFN F4 MCU. It brings most of the MCU pins out on a tiny connector.
My goal was to build a small FC that could be paired with similar sized board for different applications. So far I've designed a radio / IO board that has an OPLink radio and additional IO, and a "brushed motor" board that includes drivers for brushed motors and a 1S battery charger.
Mostly I've been struggling with where the dividing line should be between e.g. the FC and the expansion boards. The current design has a fully functional FC, with similar capabilities to the Revo (without the radio) in a much smaller package, but the number of pins exported from the MCU is pretty small, so most expansion boards would require another MCU. Another idea would be to have less connectors on the FC, and route the additional IO lines to an expansion connector so that you would have a better chance of building an expansion board that uses the FC MCU.
I would love to brainstorm some ideas on designing a small FC.