No matter how one sees the ideal liberal world, there will always be differences in size between the sovereign entities on the globe. Even in an anarcho-capitalist world there will be different sizes of the communities that people voluntarily form, although they will of course lack sovereignty in any external legal meaning we now associate with the term. So the question is: does size matter in international relations?
There are many answers to this, and I will be unable to provide them all here (or elsewhere, for that matter). There are instances where size, measured in number of inhabitants of a particular state, does not matter at all. Trade is probably the most important one. After all, it is not countries that trade but people, and if the trade is free, the nationality or location of the traders is of no importance whatsoever. This more or less still applies to the completely distorted trade situation we now regard as normal, with states continually interfering through all kinds of tariffs and non-tariff trade barriers. The real exceptions are cases where political action forbids trade, such as with the several economic boycotts between Russia and the West at present.
Size does not matter in other important segments of international relations either. In cultural exchange, there is no general correlation between size of country and quality of cultural expressions. Of course in the mainstream Western arts (sculpture, paintings, opera, symphony orchestras, etc.), it helps if a country has reached a certain level of wealth, but this is independent of size. Also, talented artists will normally be recruited from all over the world, or be able to sell their (indigenous) works globally. The same applies to sports. If one corrects for population size, many medals at the Olympics, or other championships, are won by athletes from small or middle-size countries.
The size of an economy does matter. Richer countries (in terms of gross domestic product per head) are able to direct more resources to influence international relations. Not only through military expenditures, but also through resources deployed for diplomacy, international negotiations, ‘soft power’, or otherwise. Yet a rich country does not need to be big country, neither in land mass, or in number of inhabitants. And poor countries can still influence international affairs, take for example North Korea.
Still, there are greater and lesser powers in international politics. There is not one distinguishing feature, but most of them combine a large number of inhabitants, a fairly large land mass, either a developed economy which allows for significant military expenditure or the assignment of extraordinary share of GDP to the military. Even then there are real great powers (US, China, Russia at present), middle sized powers (UK, France, Brazil, Indonesia, South Africa) and countries punching under their international political weight (India, Germany, Japan, Egypt, the Philippines, perhaps Turkey). This division is never stable, as the past decades have shown, where only the US has been a consistent top dog. Even small countries can exert significant international political influence, as Singapore shows, although the sheer lack of size will always ensure they can rank higher than the middle category .
So the answer is clear: size hardly matters at all. All countries can become more, or less important, regardless of geography, inhabitants, or economic circumstances. Different policies may cause countries to rise or fall on the “international relations influence league.” Of course, liberals will aim at dynamics caused by liberty increasing policies. That is the perpetual opportunity for liberals across the globe.