There are a bunch of different species that have red bands. The ones that show up for sale as Banded Trochus are usually either T. radiatus, T. maculatus, or T. ochroleucus. A normal adult is about 1.5 inches across for all three of these. T. niloticus gets a lot bigger, even though they may also be called "Banded Trochus," with an average adult being about 4 inches across. All of them are good algae eaters.
Cheers,
Don