This debug command will highlight all provinces that would be classified as an island under the specified threshold (as in, those that the 'Province is on an island' condition would apply to).
|A number. The threshold for classification of a country as an island. All countries below this limit will be highlighted. Unit is pixels.
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This console command would highlight all land masses that would fall under the classification of an island (as opposed to a continent) based on a maximum island size of 25,000 pixels (which is actually the default).