What characteristic makes Red Dwarfs the smallest true stars?
Answer
They are the coolest stars capable of sustaining hydrogen fusion
Red dwarfs are true main-sequence stars because they actively fuse hydrogen into helium, but due to their low mass, they do this very slowly, making them the smallest and coolest of these active stars.

Related Questions
Why are stars generally classified as 'dwarfs'?What characteristic makes Red Dwarfs the smallest true stars?In what context is our Sun often referred to as a yellow dwarf?Despite having a radius similar to Earth's, what mass do White Dwarfs retain?What primary physics event separates a Brown Dwarf from a true star like a Red Dwarf?How do Brown Dwarfs generate light and heat if they cannot sustain core hydrogen fusion?What minimum mass is required for an object to initiate sustained ordinary hydrogen fusion, becoming a true star?What stage of stellar evolution does a White Dwarf represent?How long can the lifetimes of Red Dwarfs extend?What is the basis used for classifying a White Dwarf as a 'dwarf' compared to a Brown Dwarf?