How can I find the spark plug size?

So I have a dirt bike with a spark plug that no longer... sparks. So I went to change it and I looked at the number on the plug (autolite 4194). I bought a new plug exact same make and number, and autolite 4194, and the end is smaller than the original plug... Something leads me to believe the previous owners altered the end of this plug somehow... so my question is, how can I find the size of this frankenstein plug by measuring it instead of using the number. I am honestly stumped here... any help would be extremely appreciated.