Welcome to the forums, newcomer!
The threading of the screws should be 6-32, the length should be 6mm's in length though I'd highly ask you to visit a hardware store where you can get access to nuts and bolts(and screws) and see if the screw threading is the same as the one on your AIO's radiator. You want to avoid cross threading.
Edit:
The weird part is that the screws in this review,
by TechPowerUp, shows screws(bottom left corner in the picture) that resemble M3 threading,
the same screws used for mounting your ODD or an SSD to the case. Perhaps try and see if your SSD screws work without adding any undue pressure/force to the screw driver.