I disassembled the old iron and salvaged a few parts that I could use, mainly the potentiometer and power switch.  Looking at the space inside, I should have a 45mmx40mm area for the main board and plenty of room for the panel that will house the pot, switch, display, 5V regulator board and iron connector.  I'd like to have the DC power connector in the back to reuse the existing hole and have the USB port of the nano facing an edge so I can reprogram without disassembling, but we'll see how the board lays out.