The size of ACR's adjustment brushes is relative to the screen, not to the image. So if you have a brush 50px wide, it will remain 50px wide regardless of how far you zoom in. So, to reach the smallest areas, set a small brush, zoom to 100% (or more, if required), and adjust the feathering to hide the brush's impact. However, if you need to zoom to or beyond 100% to adjust something, will the average viewer will even notice?
I generally use ACR for broad exposure adjustments, nothing too fine. It can be helpful to use a large brush to cover an area, then erase parts of it to cover the desired area more accurately.