Hi and thanks for your question.
When you enter the sysex properties (change the control to type sysex, first), you will see this box:
Use the V key on the keyboard (or with an external keyboard, type "v" or "V") in the position in which you would like the value to be placed. The bottom field--"bytes in value"--determines how many MIDI bytes the value will take up in your string.
The LSB-first option also pertains to the value: it reverses the order of the bytes in the value.