There is no hard and fast definition or set of guidelines for a microsanctuary. Of course, speaking in purely quantitative terms, a microsanctuary has a comparatively small footprint when juxtaposed with a large sanctuary or even a large farm. And the number of sanctuary residents would be much lower as well.
Beyond that, microsanctuary is as much a state of mind, a perspective on the world and our place as rescuers and caregivers, as it is about property lines and resident numbers.
A microsanctuary starts from the premise that our space and our resources, no matter how limited, often are still sufficient for us to provide sanctuary to individual farmed animals in order to prevent them from ever again being used as commodities.
A microsanctuary can be any space run by a vegan that is home to rescued animals and emphasizes their health and happiness. So someone with a rescued house rooster is just as much a sanctuary (by virtue of being a microsanctuary) as a million-dollar non-profit with hundreds of acres and hundreds of animals. By throwing out that ideal, individuals can begin to think honestly about what sanctuary means for the residents and the caregivers.
This sense of dedication to the service of rescued farmed animals, as a way to end (and help ameliorate in some way) their exploitation, is what lies at the heart of sanctuary—and on an individual level truly defines a microsanctuary.