If you removed the battery and went with adaptor power, you might be able to get a small pot in there, but it would be problematic. You would have less real estate to hit the pedal so you would step on the pot a bit. Also, having the pot on the pedal switch lid would weaken the soldier joints over time, and the pot would be very likely to fail over time.
You can get very small potentiometers that are only a few mm square, those would be better than regular guitar pots for this application, but Securb's right. You don't want to be constantly stressing those solder joints by mounting on the lid. I see two solutions:
- use a mini pot and then mount it on the side of the chassis away from the switch (might be doable, sometimes there's some room in there)
- cut a hole in the top of the pedal big enough that the shaft of the mini pot goes right through it - then you can stomp away and not stress the joints (won't be pretty though)
If it's set and forget, why wouldn't you just find the right value with a temporary clipped in pot, then mount the correct value component to the board?