Today I established the "standard" size and shape for Base18 controller boards. 86 x 55mm - almost like a credit card :) This size is derived mainly from the short side of the Base18 board, which itself is 208 x 86mm, so the controller can be flush with it in order to make a good looking package. Of course nothing prevents a controller board to be a different size, but unless there is an undeniable need for that I am planning to stick to this size and shape for now.
The board has recessed area for the main header. With appropriate PCB thickness 1.6 - 1.8mm, a standard straight header will fit in there directly by enclosing the board on both sides and will be very easy to solder too.
I made the first controller board using the new PCB template - a simple SD card interface. This one is also now in the factor for prototype PCBs.