The incorrect 136 km^2 figure comes from the slide
linked in this post. The slide (pg. 46) refers to the rendered area in two of the game's sub-regions, Novigrad/Velen (8.5kmx8.5km) and Skellige (8kmx8km), *for the purposes of the project being described in the presentation*. NOT the playable area of the game. If you do play the game, it would be evident that those figures are completely irrelevant to anything in-game. Especially given that in-game, the larger sub-region is the one that had the smaller rendered area in the above model.
In the actual game, those same two sub-regions have an actual area (as given by the in-game distances) of 28.9 km^2 (Skellige) and 15.2 km^2 (Novigrad/Velen/HOSDLC). And those figures *include* out-of-bounds areas that are past the invisible walls (which would be difficult to measure as they are not uniform).
Witcher 3 is massive; I've spent now ~1000 hours across 3.5 files and the expansions. They've struck a remarkable balance in reconciling scale and content for immersion relative to other fantasy ARPGs (not packed with cities like Skyrim, reasonable distances between villages, etc.). I'd recommend
this video discussing the brilliance of the world design. But it's not >136 km^2 (there are three other smaller sub-regions and then another huge one added with the B&W DLC/Expansion).
