It's a good question, but asking it shows that the experiment was explained poorly.

The slits aren't the reason you see an interference pattern. The slits function as two lenses, similar to a pinhole camera. That's something that usually doesn't get explained very well, you can use all sorts of lenses for this, but slits are the most basic (and crucially, glass lenses would cause an interference pattern even if light weren't a wave).

The double slit experiment is basically "if light is a wave, a slit would behave like a lens, similar to a pinhole camera. If light is a particle, it will simply be a hole without any lensing. Two slits show multiple bars, due to interference from the lenses, which means light is a wave"

Which means this works at any scale. All you need is some light in the same frequency, and something to bend it. That can be two slits, some glass, or an entire galaxy.

There are local limits of course, where the effect still applies, but things become too blurry and diffuse to make out. But that's more of a limit to your sensor than the experiment.

The interference patter gets closer and closer to a set of independent peaks when you spread the slits away. There is no single point it breaks down, and the wave behavior predicts exactly the "particle behavior" you get when the slits are too far away.