I see a macroeconomy as a ecology of inter-related plans of economic actors pursuing their own goals. The motifs, cognition, and decision-making process of economic actors are the micro-foundations of a macroeconomy. In so far as economic agents consider the macroeconomic environment while making future plans, expectations about aggregate variables are the macro-foundations of micro-economic decisions. Macroeconomics therefore is the study of the bi-directional relation between micro-economic decisions and aggregate-level outcomes.
The nature of this bi-directional relation is influenced by a variety of structural features of an economic system like the buyer-seller network between firms. These structural features emerge from the interactions between micro-economic agents. There is reason to believe that the macroeconomic phenomena and structure of relations between microeconomic agents of an economy with a hundred firms will different from those of an economy with a million firms. More is different.
As the reader may have observed, my approach to macroeconomics is somewhat different from representative-agent, IS-LM, New Classical, New Keynesian, and old-Austrian style theorizing. Therefore, I developed my own reading list to teach Intermediate Macroeconomics at George Mason University (Fall 2014 and Spring 2015). The syllabus is available here.