There are not many websites on this alone, because the different types of systems never exist without a context. I have included in the resources a website with information on generalist practice in social work, in which the understanding of the different types of systems is important. These articles do specifically reference these things, but again it'd be easier with more context.
That aside, maybe this description of the differences will help you:
Micro systems are typically client systems which might include an individual client or their family system.
Mezzo systems are larger systems that interact directly with the micro system. Examples would include schools, churches, neighborhoods, etc.
Macro systems are systems larger than mezzo systems such as governments (state, municipal, federal), larger geographical environments, a school system as a whole, etc. The size of the macro system can vary widely, as it can be as small as a single school system or as large as the federal government.
Aside from these definitions, where the lines are between micro and mezzo, or mezzo and macro is kind of arbitrary and fuzzy.