There is no set definition for this. The definitions of “strength” and of “freedom” are essentially up to each individual.
We could define strength in terms of military and economic power. We could say that a country is only strong if it could defeat most or all other countries in a war. We could say that a country is strong only if it has nuclear weapons. We could say that it is strong if it has one of the world’s top economies. It is truly “in the eye of the beholder.”
The same goes for “freedom.” We would typically say that countries like the US, are free because they protect their citizens’ civil liberties. We would say that countries like China are not free because they restrict civil liberties so much. But some people might argue the US is not free because, for example, it incarcerates too many of its people. And if we do agree that the US is free and China is not, where is the dividing line between freedom and unfreedom?
My own attitude is that strength consists largely of economic and political power. A country is strong if its people have a good standard of living and it is well-respected around the world. It is free if its people have most of the common civil liberties and rights protected fairly well. But this is, of course, a very vague definition because there is no way to be specific.