Yeah, L was really tough. According to Wikipedia, Khong Island in Laos is 18 km (11 mi) long and 8 km (5.0 mi) wide. The next largest L country island appears to be in Lithuania (45 km2).
I know the caveats state that the whole island is accredited to every country on it, but there are islands in the Ganges delta in Bangladesh that appear to be larger than the whole of Brunei. To attribute the whole of Borneo to a country that only makes up a small percentage of it's area seems illogical.