I honestly can't say that I've ever seen a trailed 5' disc...? Probably the smallest I've seen are the old 6.5-7' JD pull-type disc harrows which were mainly intended for use in front of the JD grain drill as a prep tool. These can be pulled with a Ford NAA if the gangs aren't angled too greatly with some decent results, by the way.
For the poster, a general 3-pt 5' disc will do and be the most convenient to use. There are varying brands and weights ranging from absolutely ridiculously too light and flimsy to the super heavy duty requiring no additional weight. Adding weight to a lighter one is no big deal-concrete blocks...metal weights...etc.