I chopped my 16" M4 barrel down about a inch and re-threaded it to give it a overall length of 16" including the permanently installed KK Tac brake.
What is the standard 16” barrel gas port diameter, and do I need to enlarge the gas port to a size in-between the 16” barrel and the 14.5” barrel gas port?
Thanks

No - you don't need to open up the gas port for that.  Its only when the barrel goes down to about 11.5" or shorter with only a birdcage or 3-prong on the end that you may have problems which require the gas port to be opened up a bit.  I've had a number of 11.5" uppers and all have run fine with no mods to the gas port (but the extractor springs and buffer in a full auto shorty... well now, thats a different story...)

I wouldn't mess with the gas port opening, I spoke with Tom Barr of Bushmaster about it, and from my conversation, it seem that the opening is real precise.  If you mess it up, it could cost you a bunch to fix it.