I do not think that the map is in fact 49 square miles big. Are they talking about 2D- or 3D space? Anyway, if you want to estimate the possible map size of the game, then you have to do some math with the information we have.
If you have read the December issue of the Game Informer cover-story, which featured GTAV, then you know that the map will be bigger than San Andreas, GTA IV and Red Dead Redemption combined, including interiors, the underwater world and room to spare. Now comes the question how big are the gameworlds of each of these titles?
It was confirmed many years ago that San Andreas' map is exactly 13.9 square miles big. Looking at the official map, it does make sense; the resolution is 6000x6000 pixels, which are 36,000,000 pixels in total; and if you think about it, you will realize that this are exactly 36x10^6 square meter. So, basically, the amount of pixels and square meters equals to each other. The result is 36 square kilometers, which in turn are 13.9 square miles. The official map is scaled in one meter per pixel resolution. I and others have used several methods in the past to prove that this is true, and that it is the case with all Rockstar titles, at least this console generation.
GTA IV's map is around 6.25 square miles big; that includes the entire land mass, which is about 3.45 square miles big if I remember correctly, and the water between the islands.
RDR's map size is the probably most argued one of all these games. All I can say is that it's definitely not 28 square miles big, that's very wrong. You can do in-game measurements, as well as in every other game, to calculate the size of the map. If you do it with the right methods, then the result should be fairly accurate. Using the official map of the game, which has a resolution of 7500x5500 pixels, you can try to remove the pixels of the wasted area and the water that is outside of the playable area with Photoshop. I did that some time ago, and the result was that the map is around 6.5 square miles big. If you include the inaccessible area, then the area is around 11 to 12 square miles big.
So, to make it a bit easier to comprehend; San Andreas' map is 13.9 sqm; HD-era Liberty City is 6.25 sqm; and Red Dead Redemption's map is around 6.5 sqm big, if you do not count the inaccessible area. Just to let you know, those numbers represent the in-game area of these games as 2D space, not 3D space. If you would calculate all that together, then you see that GTAV's map would be around 25 square miles big.
The fact that GI considered interiors and alike means they were most probably talking about the 3D space, which would mean the 2D-space would be even smaller. I'm not up to date when it comes to the latest estimations, but the latest number I remember was that the gameworld in V is around 20 square miles big, which is one and a half times bigger than Rockstars biggest map to date.