Are Microeconomics limited to one country? For my IB Econ internal assessment I want to comment on the falling oil prices across the globe. 

Would this be covered under Micro because it's 1 specific market? or is it international because it comes after OPEC lost it's influence on the market?
It is my impression that this would  fall in Intrernational Microeconomics, which is one of the two main subfields of Interrnational Economics. Essentially it is the modern "International Trade", where the prevailing features of the modern era (less trade barriers so less room for state policy, larger trade flows so increased degree of activity at firm-level, large private global players) make it acquire more "micro" aspects compared to the older International Trade field with its preoccupation with customs and tariffs (although these still exist of course).  

It cannot be categorized as a "macroeconomic" subject, because macroeconomics always looks things from a general equilibrium (and sometimes disequilibrium) perspective -but a general one, not partial. And a cross-section of markets of the same good is a fragmented but single market of the world economy. 

You may be talking about whole countries, like OPEC countries, but you will essentially consider them as producers of a specific product, not as national economies.