Square hole. 1/4" is too small, 3/8" seems to be too big. What tool do I need to remove this plug? Is it 5/16", or metric?

Also, can the temperature sensor be removed instead to drain the coolant from the block? That looks easier.

Thanks

Not sure...square hole....socket driver?

You use the squaredrive end of your ratchet or breaker bar. 3/8th should fit but it's likely pretty rusty.