@savage
You have about 35K of Flash space for your user application. Any system libraries that you reference in your code may already be included if they were used in the firmware.
As for SRAM a good practice is to leave at least 5K SRAM available for the stack & memory heap.
I found a small json library JSMN from https://bitbucket.org/zserge/jsmn/wiki/Home, I have not checked to see what's already been ported in the Spark IDE Libraries
UPDATE - it looks like the JSMN is available in the web IDE.