I was thinking.
The elemental countries aren't that big. I mean, a civilian crosses the distance from Konoha to wave country on foot. And that's one fourth of the whole universe!
So narutoverse is in fact an archipelago the size of Japan. Or even smaller.
And we are never shown whats is outside.
Beware scale when it comes to Naruto. It´s rather iffy about staying consistent. IIRC at one point it´s stated that Konohagakure to Sunagakure at full run for a shinobi takes about 3 days(and shinobi run FAST).
And i think there was mention that the trip to wave took 2 weeks of walking(don´t think it´s stated if that´s just travel time nor whether it´s only one way), but there´s a lot of statements that makes the place look rather fuzzy around the edges.
Also take note, that the main villages together mobilised 80k shinobi, and that there´s apparently several times more "normal" soldiers than there are shinobi. This gives some minimum sizes for what kind of lands can support those kind of forces without apparently putting all too much strain on the economy.
Japan is probably too small. But possibly not THAT much too small. And the way the land looks(more round/square rather than Japans stretched halfmoonshape), it doesn´t have to take up all that much more surface area on the planet.
My vague guess is that the land is probably twice the area of Japan, maybe three times.
Still, that isn´t actually huge or anything once you change the shape.
Then why couldn't it be an island in the One Piece world? Somewhere in the Calm Belt or in the New World.
While i´m not a fan of OP, so i only have very vague knowledge, i think it´s 100% possible. Might be a bit troublesome to fit in all the "nearby" parts that we don´t really know how far away they are, but much of those are from filler arcs anyway so they can be dropped if they really refuse to fit in.