By using map co-ordinates found on the photo mode trailer released for Cyberpunk 2077, one guy was able to interpret and overlay this information to get a fairly accurate representation of the scale of the map.
His estimations state that the size of the map is 225 squared kilometres. This is almost 87 squared miles. GTA 5 is 40.5 squared miles.
His method is best explained in his video below:
By checking his calculations I think he is only going to be a maximum of 10km2 off, so this is pretty much the best we will get until the game finally releases.