They are M4. I don't recall the length off the top of my head. jr's3800 may. I sent him a full replacement set of hex-head cap screws for his ZM throttle body. Or you can measure the thickness of the TPS in mm'*, then add another good bit for your length. The holes for the TPS are rather deep.
Hex-head will make it much easier to remove without removal of the TB. That'* a good move. I do that to all the TB'* that come through here. Star washers are also a good idea, as is BLUE loctite on the threads.