Witcher 3 isn't 136 km^2. The in-game Mini-Map provides meters so finding out world size is easy.
This is what I posted when B&W released:
This was the best analysis of the Game Awards Map mathematically - everything is trig, only guess work is tree height & Link height as reference points. That yielded a size of ~240 sq. km for the Game Awards map (smaller than X which is ~400). Obviously it's tough to reconcile how close the Game Awards footage map is with what we saw today and what we'll have next Spring.
As for TP, my math yielded 1.04 sq km (.40 square miles).
Here's my map w/grid - each square is .01 sq km, so 10x10 squares is 1 sq km. Keep in mind that the 1.04 sq km is referring to green land/blue water only. Anything in black is not counted (so the area of the total rectangle is greater).
I really highly doubt that this new map is just 12x the size of TP. The portion we saw today itself looked significantly larger than TP. I want to see a direct quote from Aonuma. It may be possible he was referring to JUST the Plateau region as being ~12x TP size.
TP on Game Awards Map overlay (assuming 1.04 sq km for TP & 240 sq km for BotW):
Scale is 1 km. Each box is 1 sq km.
Also Witcher 3 overlaid on Game Awards Map (assuming 240 sq km). Doesn't include B&W.
Scale is 0.5 km. Each box is 0.25 sq km.