Yep, what witpqs says.
Except, there's some differences between what AB has for SLs in his data and what Babes has. Babes has seriously messed with every Island in the Pacific on an individual basis, and has given the results and the algorithm to AB for him to use. Perhaps he will, at some point, but it's important to realize that AB's data and Babes data are a skoosh different. Here is what the Babes data files should do.
To amplify a bit: Islands come in three colors and many different flavors. There's three "Sizes" that show up in the pwhexe editor.
Island Size = 1; Very Small Island; All of these used to default to SL of 6,000. "Most" still do, but there's a few that get up to 10,000. We wanted to keep the default, so Michael set up the code to give a hex its listed SL, but DEFAULT if the SL value was '0'. So it's a no-brainer to make a Very Small Island have a SL of either 6,000 or (by sticking a value in the SL field) 5,000, or 10,000. But, most are still the good old default of 6,000.
Island Size = 2; Small Island; All of these used to default to SL of 30,000. "Most" now do NOT. If the SL field was set to '0' it would, once again, DEFAULT, but every size=2 island, in the game, has been vetted and individually assigned a SL. These range from 10,000 to 25,000 (with the majority at 20,000).
Island Size = 3; Medium Island; All of these used to default to SL of 60,000. "None" of them do so now. If the SL field was set to '0' it would, once again, DEFAULT, but every size=3 island, in the game, has been vetted and individually assigned a SL. For Size=3 islands, we have decided (and AB agrees) that they are large enough for the "terrain" to be the controling factor, rather than the size. Size=3 islands now range between 20,000 and 40,000.
Atoll is a terrain type, not a size. Any island, of any size, can have atoll terrain, jungle terrain, jungle-rough terrain, wooded, wooded-rough, or simply rough terrain.
Hope this helps.