They were pretty big. Far below Aerosmith, Zeppelin and Queen (and nowhere near The Stones or Floyd), I'd put them somewhere around Black Sabbath and Alice Cooper. Sabbath and Cooper put on 'shock shows' like KISS but IMO KISS put out crap music. Sabbath and Cooper were actually very talented - as bands.
I was never a KISS fan. Or a Stones fan - so take that with a grain of salt.
Eagles, Zeppelin, Queen, AC/DC, Aerosmith - all much better bands but I bet KISS sold darn near as much merchandise as those 5 put together, and there was a LOT of merchandise floating around from those 5. Sabbath might have come close, maybe Pink Floyd. Sabbath or Floyd and any one of those other 5 would have sold more merch I would guess. Even 3rd graders in 1978-1988 were BEGGING for KISS shirts. That was the success of their act.
But yeah, KISS was legit a huge band with a lot of albums and tickets sold for about 3-4 years. They died about as quickly as they rose though. The other bands either had longevity or spawned Ozzy.
tl;dr - to answer the question of act or band, the answer is "both".