When scanning barcodes in Survey123, a user has no control over the scan clipping area - it is set according to the type of barcode(s) selected in the scanning options. For Code 128 scans for example, the clipping area is square, but most of the true barcode types are rectangular in shape. If the shape of the clipping area was user-selectable, barcode scanning would be much more accurate. A square clipping area simply does not make sense for the majority of barcode types - only a few types, such as QR codes are square.
We also find a rectangular clipping are more useful for selectin specific barcodes when there are multiple barcodes in close proximity - see the screen cap attached: