[RPG] How large is Castle Ravenloft’s grid square size

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.

The 5e adventure uses the old maps for the original adventure back in the 1980's.

At that time, the squares of maps were 10 foot:

source

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.