The space is roughly 2 (W) x 2 (L) x 5 (H). I will lose some height due to lights of course. I think I can comfortably fit 2 plants, 3 is tightly squished. I have 4 right now but one is growing much, much slower than the others and only has 3 deformed looking leaves. The other 3 are equal height with nice leaves. I will say the small, deformed one sprouted a few days later than the other three but is not really catching up.

I do understand the complete dark (in theory anyway). Not sure how absolute dark would matter as in nature there is always moon light so I don't really know why a small crack of light (say through side of door jam) will hurt it any. Kind of seems like moon light, if it even made it into the cabinet.