on

If you are using the term 'an island', then certainly 'on'. If you name the island, size plays a role, and other things too. If the name of the island coincides with that of a country, I'd certainly say 'in', even if it's small. I'd say 'in Malta' but 'on Madeira'.

In Hawaii / on Hawaii, depending on whether you mean the US state or the Big Island, for example.

having lived on an island - we used to ask if someone is in the island... indicating they were not were not away - off the island...
just noticed 2008! no me bre - moonlighting as grave digger?