To view the map that will show you otherwise hidden locations, click the link: MAP SPOILER
SPOILER ALERT!: Fallout 4's entire playable map size has been a subject of considerable interest to everyone for a long time. Despite Bethesda's efforts to remain quiet on the matter, we may now have a glimpse of what kind of scale is offering due to a video posted on Reddit.
User RedBeardedT posted the link to the 11 minute and 26 second video which features a player running their character from corner to corner in the Fallout 4 map. Along the way there's visuals of a ton of varied and volatile terrain and more than a few weather effects on display. Safe to say, if you're still trying to preserve the surprise, you'll want to stay away from this one.
Nonetheless, it takes about 10 minutes and 40 seconds for the player to finish their run through the densely packed landscape. It might seem like a short time, but keep in mind the player sprints in a straight shot through most of the video. It's hard to compare that to a game like Witcher 3 which is heavily segmented and pretty much impossible to traverse in a straight line.