It's not the size that counts but what you do with it.
Please use this thread, we've had heaps of map threads. http://www.gamesas.com/topic/1542540-opinion-about-the-map/
you can't determine playing area just by running across from one side to the other, most of skyrims map was unplayable area, the base of the mountains took up over half the map, fallout 4 has more playable area, maybe twice the area, it does have water but so does skyrim, skyrims map was deceptive when it came to actual explorable area, you were funneled around by the mountains in mostly valley areas, you could get to the tops of a lot of the mountains but the footprint they took up was huge it covered over half the map area, remember bethesda said fallout 4 has twice the assets as skyrim so as huge as skyrim was with all the dungeons, quests, areas etc, fallout 4 is double that.
Yeah this doesn't really matter since there's too many factors. And someone with noclip on went through the mountains in a direct line and did it in under 12 minutes. Can't really say much without knowing speed also.