Character wise, this is the list of already established characters (from the movies, etc), that have been taken (OCs don't count because they're already original anyways):

Godzilla (Me)
Godzilla Jr (Me | NPC)
Kiryu (Me) (Me | NPC)
Gipsy Danger (Me)
Otachi (2b3heart)
Leatherback (2b3heart)
Biollante (spiderfucker)
SpaceGodzilla (spiderfucker)
King Ghidorah (ianzerep)
King Kong (Holy Soldier)

Size wise, the limit is around 100 meters (with Ghidorah (or Biollante) taking said title as one of the largest kaiju around). And whats is limited depends on what you make.