Nematodes are generally not that large. One thing you can do is to "save" the sample (not put in a preservative solution) and see if it disappears. The point of this exercise is to determine if it is indeed a parasite
or plaque coming off the colon walls. Worms generally disintegrate over a few days where if it were plaque then it would not disintegrate.
Tapes have distinctive shapes, e.g. they are not smooth but looks like a Michelin man. Typically people see white rice particles in their stool which has a high probability of being tapes, the white rice are actually the eggs they lay. These are the only eggs that can be seen with the naked eye. Nematode eggs (ova) have to be viewed under the microscope.
Worms have distinctive shapes meaning uniform and not irregular. They are cylindrical and have tapered heads/tails. They may come in partial pieces (not whole). Roundworm is fairly common but not more than 7" or so in length.
I would be curious what treatments you tried.