Because I was playing D&D Pool of Radiance, and it uses the same size map grid that M&M did.

I'm wondering if most RPG's use it as a universal size? Or just a coincidence?

I remember that Wizardry (1-4) and Bard's Tale 1 and 2 used 22x22 maps.

In any case,one advantage of 16x16 maps is that the squares in them can be indexed by a single 8-bit integer, which is easier to work with than a 16-bit integer on an 8-bit system, particularly if writing in assembly language.

Fate: Gates of Dawn for Amiga had surprisingly large maps for its age. CRPGAddict speaks about it here: http://crpgaddict.blogspot.ru/2016/12/fate-land-and-sea.html (warning, spoilers in that link)

The Amiga was 16 bit, though.

Ok, Oubliette had maps with size 25x25. And it is theoretically the first ever CRPG.

Oubliette was made for the PLATO system. I think that was quite a bit more powerful than the first generation of home computers. Not sure how many bits it was, though.

But It was ported to IBM PC for DOS in earlier 80s.
There was always a way to make large maps.
Also 16x16 map can take around 256 bytes of memory. Most of the memory and CPU was used for graphics even as primitive as they were back in 80s.
Even now, one can make grid-based first-person-view game with size of maps 1000x1000 and more, but we rarely see anything higher than 32x32.

Yeah, it's always easier to make levels that are square and with "round" numbers of squares.
The predictiability of the size of the levels actually annoyed me a bit with Bard's Tale 1-2, and the Gold Box games.
I think Ultima Underworld 2 was the first game I played which had very variable level sizes.

So, you played Ultima Underworld 2 before you played Bard's Tale 3 (which has variable level sizes)?

I remember playing Bard's Tale 3 long before computers were technically advanced enough to support Ultima Underworld 1, let alone 2.

As PetrusOctavianus suggested 16 is "programming number" because it's power of 2. Basically it can be stored on 4 bits of memory.
So it's not coincidence, just technical quirk.

Speaking of Oubliette, from looking at maps online, it appears that the DOS version has one level where you can go out of bounds. When you do so, the game is treating other areas of memory as if it were map data, and that the game can supposedly become unstable.

This is in contrast to most other early CRPGs where, if you go off the map, you simply end up on the other side.