The original Naruto Forums calculation seems to be unavailable, and there are some problems with it. Their size for Papaya Island was wrong, big time.
Since the link is dead, I forgot the exact size, but if I remembered it was 30 kilometers or miles. If it was miles, it would be about the size of Rhodes, if it was kilometers... Well, Malta would only be comparable.
Now, even in maps of just Europe, Rhodes and Malta are extremely tiny and hard to see. Which would be hillariously inconsistent.
Even in the world map, Papaya Island (The westernmost island on C3) is highly visible, it is pretty dang big (3rd biggest island on the map)
Official sources even say it is pretty dang big for island standards.
Now, I know, I know. They calculated it to be 30 km/miles, but, calculations only take precedence over word of god when the WoG is inconsistent. If the size of the island was never stated, and there was no map to compare to, then the calc could be used, but we have a statement and on the maps it is shown to be big, a calc (Which we cannot see anymore no less) that would make it so small should automatically be thrown out in favor of WoG in this case
Now, for the calc
The map is 2004x2000 pixels, which is to scale with the earth
1 Equatorial degree = 5.566666666666667 pixels = 111.319492 kilometers
1 Meridional degree = 5.555555555555556 pixels = 111.132944 kilometers
I need to find the degree distances first
The island at its closest is 55 pixels away east from the center, or 9.880239520958083 degrees
It is 696 away south, which is 125.28 degrees
Its easternmost part is 148 pixels east, or 26.58682634730539 degrees
Longitude = 99.88023952095808
Latitude = -35.28
Longitude = 116.5868263473054
116.5868263473054 - 99.88023952095808
= 1859.768758562876 kilometers
I get 1515 km for the size of Papaya Island
The island is 872 pixels which equals 1515 km
The exposion was 475 pixels, or 825.2580275229358 km
Using the SD.Net nuke calculator, I get 3.98999 Teratons of TNT or Small Country Level