While these terms are most commonly used in introductory textbooks to describe three-note chords or four-part harmony (with bass treated as a sort of separate part of the texture), I've definitely seen the terms used with other types of chords on occasion.
The implication, to me, with "close position" is that all notes are clustered on adjacent chord tones. As noted, the bass is sometimes treated as independent, so its location doesn't matter. Also, I'm pretty sure I've seen the term used to refer to a subsection of a texture, e.g., "these three instruments are in close position," where other instruments may be further distanced.
So, it's not necessarily always applicable to an entire texture. It's referring to voices that are directly placed on adjacent chord tones. If there's a gap where a chord tone is skipped between adjacent voices, those voices -- however many there are -- are no longer in "close position." It seems reasonable to extend this definition to more than three voices, again as long as there are no gaps.
Anything else, I suppose, is by definition "open," which is sort of how that term works with 3 or 4 voices anyway. These terms certainly aren't meant to express great nuance in possible texture/voicing. There is also an informal sense where "open" means something like "spread out," again referring to some group (or subgroup) of voices that tend to have greater gaps between the voices, rather than being on adjacent chord tones.
When referencing an entire large chord with 5 or more notes, these terms aren't generally that useful. My sense in that case is that they can occasionally be used to refer to subgroups of voices within a larger texture. The only time the terms might be useful for the whole chord is if all the notes were on adjacent chord tones (and thus all "close").
But the problem is once you get textures like that, you inevitably start to have doublings at the unison as well as at octaves, and that seems to disrupt the whole classification system -- wouldn't three or four voices in literal unison be the "closest" possible? Yet, I've never seen the term used that way.
I don't know that I can come up with a source for the above definitions, as it's mostly informal usage when it occurs outside of an introductory textbook. But I could dig around and find some examples if necessary.