I have a list of countries, some with a very long name:
You've simply got it backwards. Specifying a minimum width would make the select menu always be at least that width, so it will continue expanding to 90% no matter what the window size is, also being at least the size of its longest option.
You need to use max-width instead. This way, it will let the select menu expand to its longest option, but if that expands past your set maximum of 90% width, crunch it down to that width.