single stranded nucleic acids do not trap much ethidium bromide so quite a lot is required for agarose visualisation. PAG gels can be silver stained which is much more sensitive and bands are tighter so may be preferable if you do not have much material . How much are you loading in each track?
yes long SSDNA or rna will form secondary structures of DSdna which intercalate more ETBR so size definitely makes a big difference to visibility but the amount loaded may still be a factor