> OK, so your reasoning is with respect to maximal size that one voxel can take
> once rescaled and that rescaling won't probably exceed an upsampling of 4,
> which seems reasonable.

Yes.

> I was more concerned about blocks of NaNs in the data that are larger than
> 4x4x4, even if there is no rescaling. But I don't know if this is something
> that can happen

This might happen, and it would be a problem if we actually tried to do
inpainting of missing values in the NaNs, but we are not doing this. We
need to extrapolate only to avoid border effects near the NaNs.