This is really just a minor point I've been thinking about, so please don't waste much time answering it -- but I couldn't find a great answer anywhere else.

Here's the question: We often throw out statements like "Quantum effects generally don't matter much (or cancel out, or however you care to phrase it) on the macro scale -- at non-quantum sizes, classical physics works well enough." But what is a good rule of thumb for the spatial-scale threshold where you'd start to care about quantum effects?

When I search around for answers, I mostly get articles about (what I'd call) edge cases like superfluidity where quantum effects ARE visible on the macro scale. That's not what I'm looking for. I'm thinking more about how small a box you need to draw around your system (consisting of ordinary matter that is NOT in a superfluid or other macro-quantum state) before you need to invoke non-classical physics to adequately characterize what's going on.

I recognize these are loosey-goosey terms and I'm not looking for a precise answer. Obviously a specific answer would depend on lots of things -- whether I'm talking about capturing 90% of the variance of the system vs 99% vs 99.999999%, what kind of stuff is actually IN this system, etc. I'm just trying to get a rough picture in my head of the scale, within 2-3 orders of magnitude, abstracting over as many of those details as possible without making the question totally useless.

I did see that this question was asked in /r/askscience before, but the top couple of answers -- while good answers in some sense (and the same kind of answers I often give laypeople about my own work) -- do kind of dodge the question. One of the further-down answers says "A good rule of thumb: Quantum mechanics becomes 'important' at scales such that the de Broglie wavelength of an object is around the same order of magnitude as its size." That's more the kind of answer I'm looking for (if it's correct) -- but what's an example of that scale? I don't know quite enough about matter waves to work out the math easily for myself.

Thanks in advance...