lpetrich
Contributor
Star Trek has a planet-classification system that was likely inspired by the spectral-type system of stars: OBAFGKM. From several sources, one can piece it together. Planetary classification | Memory Alpha | FANDOM powered by Wikia, Ex Astris Scientia - Planet Classification, 
Star Trek planet classification
[0707.2895] Mass-Radius Relationships for Solid Exoplanets has some calculated-structure curves in its Figure 4. The largest sizes are reached for masses around 1000 Earth masses (3 Jupiter masses). H-He: 11 Earth radii, water: 5 Re, rock (MgSiO3): 3.5 Re, iron: 2.7 Re. Jupiter is about as as big as a planet can get without something puffing it up. So J is as big as it gets, and I, S, and T are not physically possible without some artificial puffing up. But if "size" refers to mass, then I, S, and T are possible, even if smaller in space dimensions than J. They could fade off into brown dwarf stars.
One of the sources is Mandel, Geoffrey (2002). Star Trek Star Charts: The Complete Atlas of Star Trek, and that is of doubtful canonicity.
Star Trek planet classification- A: Gothos
- B: Mercury
- C: Pluto
- D: Small airless rocky planetoid: Earth's Moon
- E, F, G: proto-Earth-sized
- H: Earth-sized with an arid surface
- I: Gas supergiant, larger than J
- J: Gas giant: Jupiter, Saturn
- K: Earth-sized, cold: Mars, Mudd
- L: Earth-sized, marginally habitable: several
- M: Earth-sized, habitable for humanity: Earth, numerous others
- N: Earth-sized, hot: Venus
- O: Covered with water
- P: Covered with water-ice
- Q: Very changeable due to an eccentric orbit or star variability
- R: Rogue planet (one that orbits no star)
- S: Gas ultragiant, larger than I
- T: Gas ultragiant, larger than S
- Y: a "demon" planet, one with very nasty surface conditions
- X, Z: other "demon" planets
[0707.2895] Mass-Radius Relationships for Solid Exoplanets has some calculated-structure curves in its Figure 4. The largest sizes are reached for masses around 1000 Earth masses (3 Jupiter masses). H-He: 11 Earth radii, water: 5 Re, rock (MgSiO3): 3.5 Re, iron: 2.7 Re. Jupiter is about as as big as a planet can get without something puffing it up. So J is as big as it gets, and I, S, and T are not physically possible without some artificial puffing up. But if "size" refers to mass, then I, S, and T are possible, even if smaller in space dimensions than J. They could fade off into brown dwarf stars.
One of the sources is Mandel, Geoffrey (2002). Star Trek Star Charts: The Complete Atlas of Star Trek, and that is of doubtful canonicity.
