I scaled them based on the most common ball chain size. But that is difficult as ball chain actually come in all sizes.
I don't think they have to be the same size as real world dog tags, which is 2" long. After all if this were hanging on your power armor they would need to be larger anyways.
Also, at 75mm long, the fonts on the front as just 4pt. If it were 51mm long, then the fonts would be use 2.5pt which is difficult to even laser-etch that small.
If they have a battery and LED inside, they basically have to be that big at minimum. If it were made without electronics then it can be any size.
I haven't done a 3D print yet, I am waiting on my sister to tell me how complex she wants to make these.