It's a 13mm. I've replaced it once. In Ford terms, it might be a 1/2 since it seems that some bolts are metric and some are SAE. The 2 are interchangeable, but there is a slight difference in size. Go with whichever one has the best fit.
Yeah I found out last night thanks a 13 mm will fit as well. I also found out BE CAREFUL the thermostat doesn't fall out of the housing when you put it in. Coolant tends to kinda gush everywhere when the thermostat is between the seal and the housing not in the seat and kinda makes you have to do it twice.