Calculating the LSB is fairly easy. You will need to know the resolution of the A/D conveter in use i.e. 12bit, 16bit, or 24bit. You will also need to know the full scale (FS) range that you want to calculate the LSB for. Below is an example for a -5 to +5 volt input range with a 16bit A/D converter.
FS Range = 10 , 16bit resolution = 65,356 bits.
Calcualtion is as follows: 10/65536 = 0.0001525 volts/bit (153 uVolts/bit)
So for a +- 5 volt input range on a 16 bit DAQ device your LSB would be 153 uVolts.