I’ve read that the wavelength of macroscopic objects is so small that it’s effects are negligible so the object can be described with classical physics, and that decoherence causes quantum effects to dissapear on a large scale so classical physics takes over. Which one is it? Or is it both? (For a layman).
For a layman ...
The two are related.
The 'wavelength' is to do with the wave function. For quarks and other subatomic particles the wavelength - the amount of space that they are considered to exist in - is considerably larger than the 'size' of the particle (if we can really say it has a size).
So using measurements of scale that will be familiar, if we know that an object that is 1mm wide is somewhere in a space 1m wide we can see that the effect is significant, we don't really have any idea where the object is.
For a macroscopic object the individual particles exist within their own wave functions but the scale of the wavelengths are so small in comparison to the size of the object that the effect of the overall object is negligible. So if we have an object that is 1000km wide and it exists 'somewhere' in a space that is 1000km and 2 mm wide we are pretty certain of where it is.
Quantum Decoherence is an effect from the way different systems interact with weach other. Particles in isolation do not experience any decoherence, the more they interact with others the more decoherence happens. A simplistic way to think about this (for the layman) is when the wave functions (wavelength) overlap each other then they start to affect each other.
Larger systems exist with complex interactions between component parts. So the fact that a body is larger necessarily means that the decoherence becomes increasingly significant.
You asked for an answer 'For a layman' - I hope this helps!