There is no factual answer to this question. Everyone's opinion will vary. They can survive in a 8x8' enclosure, but I don't think that is large enough. I would like to see an adult in something 50x50' at a minimum, but would prefer something much larger. I keep juveniles in a 30x30' enclosure.
Also, I'm not sure what you are asking here. Do you mean to ask how much space they need, or are you asking how big of a space will be needed to grow enough food to maintain one sulcata?
Ficus leaves are toxic and yes, it could be a problem for a sulcata.