How do I check and/or change the sample size of the eyedropper in Adobe Camera Raw

In ACR and LR, the sampling is always reads 5-by-5 screen pixels at zoom levels of 100% or less, so it may give different values at different zoom levels. When you fit the entire image into Camera Raws preview, you are sampling an average of a fairly large number of pixels - the exact number depends on both the camera's native resolution and the size you've chosen from the Size menu in the Workflow controls. At zoom levels greater than 100%, the sample size is always 5x5 image pixels. But WB is zoom level dependent. So if you want to average over a larger area, zoom out.