I am tossing in printf/iprintf from newlib into one of my FreeRTOS tasks (using version 6.0.0). Is there any way of knowing how much stack this will consume, or am I left to the measure/modify loop until things stop behaving randomly?

Try the following links

http://www.freertos.org/Stacks-and-stack-overflow-checking.html
http://www.freertos.org/uxTaskGetStackHighWaterMark.html