One way to find out for sure:
Grab a bolt that uses a 14mm wrench and one that uses a 12mm wrench.
Stick the heads into the plug and whichever one fits properly is the size you need.
Slightly snug is OK, loose is not as the brass is completely unforgiving when it comes to reefing on it with an undersized allen wrench and it'll end up ruining the hole and then nothing will fit right.
Looks like you are on the right track. It is always a good idea to make sure you can remove the filler bolt before you drop the oil. Nothing like that sinking feeling when you have a nice empty diff and a filler bolt that is stripped/frozen. Learned that the hard way.