The amount of text is arbitrary and the line need only be shorter than 256 characters. Lines over about 80 characters may not look good in the output. Note that the operators at some telescopes, including the VLBA, do not pay much attention to what is in the files. Cover letters of any type are not very effective. Your observation should be described well by the parameters that actually control telescope operations.
Cover letters can be effective for observatories where some operations have to be done by hand.
No argument required or useful. Only use preceeding the cover letter input.