There is a property node in LabVIEW that will let you read the maximum available First In First Out (FIFO). This property node will let you verify the size of the FIFO listed in the specifications or data sheet.
First create a task in MAX that will use the desired channel for which you would like to measure the FIFO (e.g. Analog Input, Analog Output, Digital Input or Digital Output). Then wire a task constant with this task selected into a DAQmx Buffer Property Node.
This property node can be found on the Functions Palette under Measurement I/O»DAQmx - Data Acquisition»DAQmx Advanced»DAQmx Constants & Properties»DAQmx Buffer Property Node
. Select the Onboard Buffer Size property through either the input or output categories. Finally, change the property to read and wire an indicator to it.