The Area box determines how far from the brush strokes is affected by the chosen style. It's not the same as brush size, but you'll have to try both to see. I recommend using a small brush size and varying the Area setting. Fidelity can be set by the same type of slider as Opacity. This setting determines how much the color will vary from the original.
Lower values mean more variation.
The Spacing value limits where the art history brush will be active. In portions of the current image that are not very different from the history state that you are painting from, a high spacing setting will not allow any changes. Only parts of the current image that are very different from the chosen history state will be affected by strokes. A low Spacing value will allow painting on all parts of the image. Shown below , is a portion of the art history brush's options bar showing the brush stroke drop down menu activated.