Courland wasn't a colonial power, it just had colonies. To be a "power", you need to have those colonies be more than just a few tiny settlements in a few islands.
So not the best comparison.
There's no set definition of 'power', which is why the question is restated simply as 'what if Brittany managed to colonize parts of the New World'.
The point was that the difference between a country with colonies and one with out is almost entirely desire. If a country's leaders want to have colonies they will, even if like Italy they have to go to war to get them, even if like Sweden they are tiny and undefended, even if like Scotland the country can't afford them, even if like Courland or Brandenburg the attempts don't stick, even if like malta they have to borrow someone's elses.
To get a European country colinising the new world, you just need them to be independent, have sea access and have a leader who wants to.
If a country as far away from the north sea as Courland can attempt to build colonies in Africa and the new world than obviously Brittany (which is about the same size but closer) could too. That isn't me saying that Brittany = Courland but merely making a point about the role personality plays in these things, given that courlandian colinisation was just one man's dream.