Maximum size (as vertex count) 8161 x 8161. In 3rd Person Template it looks like 50 - 80 cm between two vertices comparing to character size. If we say 61cm then maximum size is going to be 5kms x 5kms which is about 25kms squared. But I think default scale is very detailed. You can scale it. Scaling up 10 times up means 2500kms squared. So you can achieve Skyrim map easily with better detail. But it is connected with your world design. Btw this is my opinion not a true reference.