[cowlinator (OP)]

If a large brown dwarf gains mass and becomes a small red dwarf, does the radius increase, decrease, or remain (roughly) the same?

[Kryptid]

This is an interesting question that I have wondered myself. Brown dwarf stars are not much larger in diameter than the planet Jupiter, despite being much more massive. This is because the extra gravity compresses the gas that it is composed of to make it increasingly dense as its mass increases. Paradoxically, a true red dwarf star similiar in size to the planet Saturn was discovered recently: http://www.cam.ac.uk/research/news/smallest-ever-star-discovered-by-astronomers. It seems paradoxical to think there is a star which is both heavier and hotter than a brown dwarf yet still smaller than one. I don't know how to explain this myself.

[evan_au]

Brown dwarf planets the mass of Jupiter up to 90 times the mass of Jupiter don't change much in diameter.
See: https://en.wikipedia.org/wiki/Brown_dwarf

Red dwarf stars go down to the diameter of Jupiter (about 10% of the diameter of the Sun), and mass down to 100 times the mass of Jupiter.

Below 100 time the mass of Jupiter, an object is considered a brown dwarf; spectral data indicate that it is too small to undergo fusion, which would quickly consume all its lithium ("quickly" in astronomical terms).
See: https://en.wikipedia.org/wiki/Red_dwarf#Description_and_characteristics