The answer to whether family therapy is a meso- or micro-level intervention is that it is both.
To truly understand this concept, and understanding of systems theory is necessary. It sounds like the question is coming from the prespective of Broffenbrenner's ecological systems perspective.
Family therapy can be a micro intervention. It can deal with internal, psychological items. It can work within the microsystem of a family.
Family therapy is also a meso-level intervention. It can deal with the interaction between different sub-systems of a family (a married couple, parents and a child, etc...) and any larger system they may interact with (school system, medical system, court system, other family systems). Family therapy is a meso-level intervention because its primary consideration is not necessarily the content of the problems that are presented, but rather the process in which the given system being examined is using to address or maintain the presenting problem.