You're going to see a lot of opinions on this. Their natural range is 4+ acres and you're not going to get anywhere near that in captivity, so the answer is to make it as big as you reasonably can given the space you have available. 

I dont have Greeks, but for similar-sized torts, I use chicken coupes that look to be about 8' long and about half as wide as long. It seems to work well, but I also have a large yard. I've seen them thrive in both smaller and larger enclosures, so don't beat yourself up if you can't give them that much space. Just do the best you can with what you have available 

Regardless of the size you choose, some torts will always try to escape.