What does microglia process length tell us?
Part of this question is due to the function of microglia in the healthy brain (as we currently understand it).  The general idea is that microglia in a healthy brain are in "surveillance mode," and have long, thin processes that extend of the cell body to monitor the environment.  It would be like beat cops splitting up and walking the streets when no crime is reported, they're just monitoring one large area by spreading everyone out.  However, when some insult occurs and microglia are activated (which just means they get into a mode responsive to some insult or injury), which results in them having shorter processes and enlarged cell bodies to focus on the damage done in their immediate vicinity.  This would be like one of the beat cops seeing a crime, then calling for back-up.  Now the focus is on the scene of the crime or site of damage.  Therefore, if your microglia have long, thin processes, everything is pretty much normal because these microglia are not activated.  However, having a lot of microglia with short, thick processes indicates that they are activated in response to some injury or insult.  This is why a common practice is to use microglial process length as an indicator of microglial activation and, in turn, some form of damage to the brain.  

In shorter terms:


Long, thin processes = lack of microglial activation and little to no brain injury
Short, thick processes = microglial activation, indicative of brain injury


My source is just my experience in neuroscience and reading these types of papers, but there's more information on microglial activation here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5431479/ which includes some discussion of process length and morphology.