You can configure the data width for acquisition operations using the following property or attribute.

LabVIEW Property C Attribute
Advanced:Data Width NIHSDIO_ATTR_DATA_WIDTH

Valid values for data width vary by device.

Device Supported Data Widths
NI 6541/6542 1, 2, 4
NI 6544/6545/6547/6548 2, 4
NI 655x 1, 2, 4
NI 656x 1, 2