I'd been wondering about that myself! So, I went and did a bit of math.
To start with, I want to be up front with what assumptions I'm making. First is that the Lands Betwen follow a 24-hour cycle like earth that has been sped up for gameplay reasons much like the map has been shrunk for gameplay reasons. Second, travel and time are linked - longer distances take more time to traverse. Finally, we assume that other players are correct in measuring the Elden Ring map at 30.5 square miles.
The day-night cycle in Elden Ring takes about an hour to complete. If time passes 24× the speed in Elden Ring, then I feel like it isn't a far jump to assume that distance is 24× shorter as well. To make the math simple, we take the square root of 30.5 and multiple the result by 24, and then square that to give us the size of the Lands Between if time is used as a reference.
Tl;dr: 17,568 square miles, or about the size of Estonia.