Medium mass stars can be up to 8 times the mass of our sun. High mass stars are larger than 8 times the mass of our sun. The smaller medium mass stars, between about 1 and 4 times the size of our sun, will eventually become red giants, before becoming white dwarfs and eventually cooling through brown dwarfs to black dwarfs. Larger medium mass stars ranging between about 4 and 8 times the mass of our sun will become neutron stars. High mass stars will become supernovas and then black holes. By the way, low-mass stars have insufficient mass to generate enough heat to go through a giant phase.
Note: The sizes listed are approximate and may vary depending on the model used.