I'm going to be dming a Dungeons & Dragons 5th edition Ravenloft campaign here shortly. I was curious as to exactly how big the squares on the castle ravenloft map are supposed to be. I would assume they're five feet squared but it almost seems like the castle would be bigger than that. It almost feels like each square would a 10 foot by 10 foot. I was looking on the map and in the book, but I still can't find anything that tells me exactly what it is.
They are 10-foot. You could either use them as-is (and lose the feeling that the castle is big, or redraw the maps splitting each square into four.
At that time, the squares of maps were 10 foot:
D&D had its roots in wargaming. The older books used to tell you to measure out movement in terms of inches, with 1 inch of movement representing 10 feet of simulated in-game dungeon.
You had 10-foot squares because of that 10:1 scale conversion. This was also important because then you could convert 1 inch of movement into 10 yards while out in the wilderness, compared to 10 feet while inside a dungeon.
Also, there was the temporal component to consider: because the smallest representation of time, which was the combat round, was 1 minute long, you sort of had to have larger squares because you didn't need the fine-grained granularity of 5-foot-squares if the lengths of time assumed to happen within that round was so long (1 minute) in the first place.
As for how many persons would fit in a square, older D&D editions suggested that you could have two, or as many as three, man-sized humanoids fit across a 10-foot corridor.
I'd also like to note that it wasn't exactly 3rd Edition that caused the shift in scale. The Combat and Tactics supplement for late-cycle AD&D 2nd Edition, part of the books that are sometimes called "AD&D 2.5e", was the one that started explicitly tracking squares, started using 5-foot squares instead of 10-foot squares, and reduced the combat round to 10-15 seconds rather than 1 minute segments.