Never calculated AC4 but remember that relative to the real world, even the largest game maps are tiny - so a ship traveling at normal ship velocity would be able to traverse a [relatively large game map] rather quickly.
I might take a crack at it next time I play, but till then this is the best we have:
Pulled from Ubi-boards
Easily largest AC map, but remember that most of the landmass can't be traversed. All the land on the large islands that contain Havana/Nassau/Kingston are meaningless. You can visit the cities but they've got hard natural/man-made borders and the other sites are typically on the shore or don't allow you to stray too far from it.
Couple of the mid-size islands let you go deeper, but are restricted with linear pathways or mountain walls.
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JC 2's in-game map has proper measurements if I remember correctly. Panau is roughly 20 miles by 20 miles with all landmass + water, so 400 sq miles. Pretty much all of that can be traversed, but it's a little bland if you don't like blowing shit up.
Looking at the Steam sale for JC3.
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With Witcher 3, I'm somewhat certain that the 136 sq km figure came from the rumor mill and not directly from CDPR. CDPR chose to put actual in-game measurements (in meters) so it doesn't make much sense that they would quote inaccurate numbers.
FWIW, I confirmed that the mini-map measurements were in meters via
angular diameter calculation (TW3 pre-B&W had a 60 degree FOV, which is when I verified).
We can do the same with BotW but we need particular circumstances to do so.