It really depends on how you choose to define the granularity. I prefer my commits to all be buildable at least, and ideally also pass the tests (so new code + new tests for it should be in the same commit); but that is subjective and there are valid arguments for smaller commits that are not necessarily isolated like that.

I can't think of any special tools you'd need for this? A git history visualizer with the ability to easily diff and squash ranges helps, but this is nothing special for pretty much any modern IDE. At the end of the day, you just look through the WIP branch history and identify chunks that span multiple commits but logically represent a single change towards some goal.