Go for 3/8" if it's just the PCB, 1/2" if it's also holding down a stack extension. Also, sometimes it's useful to have longer screws. I just worked on a Lionel Challenger, and half the smoke unit screws were stripped in the pot metal bowl! Since it looked like it was never opened, I suspect it happened at the factory, runaway power tools. I just used some longer 4-40 screws to engage the remaining threads and all was well.
This is also why I use screwdrivers with a 1/2" handle for most of the work, makes it harder to get carried away and strip screws. I've lost count of all the plastic shells I've repaired when people either strip the screws or twist the mounting posts off! It's plastic people, you don't need 100 ft/lb of torque to put them back on!
