Honestly, judging by just how big those city-domes must be, I figure that Lavos is probably about half as wide as Manhattan. All things considered, that's abso-fucking-lutely HUGE. Lavos is big enough to eat Godzilla like a hotwing, were that the sort of food Lavos was into. As we all know, being the planet-sucker it is, such is not the case. I remember seeing somewhere that there was a debate about the size of the Black Omen, and it was, through a bit of estimation, that it was about as wide as Manhattan island, and the Lavos sprite that's used during the eruptions is about half that particular dimension.
Very, very rough estimates, but it makes the visualization all the grander, don't you think?