The minimum is about 80 Jupiter Masses. Smaller than that and you can't start fusing.

Maximum size is harder to answer. It's determined by the Eddington Limit. Which describes the luminosity at which radiation pressure is enough to overcome gravity for a certain mass.

It's thought that the maximum mass of a star is somewhere around 150 solar masses, but there's some evidence to contradict this, as we've seen a handful of very old stars with masses or luminosities higher than they should be.