Frost plug size

[SUB ZERO CAR FANATIC]

I was wondering if anyone might know the size of the frost plug, right under the alternator and under passenger side exhaust manifold. It has a hex hole that appears to be there so the frost plug can be unscrewed, this is related to the block heater but since there is no info on the block heater available I thought maybe someone might know the diameter by chance then maybe I can find what I need by dimensions instead of a part number since there does not seem to be any info out there on that.