Where to get '--fldMean' and '--fldSD' for single-end Salmon run

I need to run single-end bulk-RNAseq salmon - based alignment and I am not sure what I should supply as --fldMean and --fldSD which seem to be crucial for the single-end experiment alignment. Can they be somehow derived from the FASTQ files that I have, or should I be provided with them by someone who did the experiment?