This idea intrigued me quite a bit. I loved the thought of seeing Red Dead's environment 100 years in the future. So I decided to do some experimenting and measure the size of Liberty City and Read Dead's Blackwater to compare them size wise.
This is a map showing the relative size of the city and extra space. By comparison, Liberty City takes up about 2/3 of the Red Dead Map. West Elizabeth and Tall Trees is about the exact size and shape of Broker/Dukes.
Here it is: