NodeMCU ESP12 Voltage I'd like to power an NodeMCU off of four 1.5v batteries.  That's 6v.

Depending on the source I've read that 5.1v will fry the board and, just a few clicks later, that the board will take up to a 20v input.

What is the real, practical input voltage range for an NodeMCU?  Even the spec sheets seem to disagree.
I edited your question and changed ESP-12 to NodeMCU, based on the title and tag. 

5.1 V will 'fry' a plain ESP12, but the NodeMCU is development board with esp8266 and the board has a power regulator for USB power or Vin pin. The parameters of the regulator depend on the regulator used by the manufacturer of your board.