In order of height: The Lunette is the tallest from the rim to the base of the cup. (the stem has been trimmed on this cup) The Mini Classic Meluna is the shortest cup from rim to base.
Here is an alternative height comparison. The difference between the Lunette and Meluna is emphasized in this photo. So if you are questioning what cup you should get regarding length, consider the vast difference between a Small Lunette and a Mini Meluna.
In order of diameter (from the outside): The Lunette has the widest outer diamter, and the Ldaycup and Mini Classic Meluna have the same diamater. The Small Soft Meluna has the smallest diamter.
The Small Soft Meluna fits inside of a Small Lunette, and the difference in diamter is emphasized.
The Small Soft Meluna also fits almost entirely into the Small Ladycup and the difference in diameter is emphasized:
In order of the rim firmness: This is only a matter of my opinion, and not quite as objective as my other observations, but in my opinion, the Ladycup has the firmest rim (because it is thicker), while surprisingly the Lunette has a pretty soft rim (its not as firm despite having a firmer body overall), and the Small Soft Meluna has the softest rim. Consider these comparisons when getting a cup because you may have trouble popping open a Soft Meluna which will make creating a seal more difficult.
Other observations: I think it is odd that the Mini Classic has a thicker rim than the Small Soft. Perhaps the rim on the Small Classic is thicker and therfore more firm? I do not know because I have not seen a Small Classic Meluna in person. Any comments regarding that are welcome.
Interestingly, the small Ladycup has a different consistency throughout its body. The half portion closer to the stem is softer than the other portion closer to the rim where the holes are located. The Lunette has a more consistent feel throughout but is still not 100% consistent. The portion of the Lunette closer to the stem is softer in comparison to the portion closer to the rim, but the portion closer to the rim is not as firm as the Ladycup. I would say overall though that the Ladycup's body is squishier than the Lunette's body, which is why I prefer it over the Lunette.
Side by side the Soft Meluna and Classic Meluna do not feel that much different in firmness, but when compared to the other cups you can feel how much softer the Soft Meluna really is. I had trouble opening up the Small Soft Meluna so I am not using it, but I did not have trouble opening my Mini Soft Meluna (not pictured in this post since I'm using it now :P). I think the firmness of the rim really makes a difference. The rim on the Mini Soft Meluna that I'm using now has the same thickness as the Mini Classic Meluna, which is thicker than the Small Soft Meluna. Even though the bodies of the Mini Soft and Small Soft feel the same, the thicker rim on the Mini Soft really helps with popping open the cup, despite the Small Soft being bigger. The firmness of the bodies of the Mini Soft, Mini Classic and Small Soft are consistent throughout their bodies, unlike the Lunette and Ladycup.
Recommendations: Out of these cups I would recommend getting either a small Ladycup or a Mini Classic (or Mini Soft) if you are concerned about length. I would not recommend getting a Lunette because that little bit of difference in size, even though its hardly anything at all, really makes insertion and opening very uncomfortable for me. The Small Soft Meluna is just far too soft to open, but oddly enough the Mini Soft Meluna is not, perhaps because the shapes are different and the rim is firmer on the Mini soft, so I would choose the Mini Soft over the small soft. If you want to get a Meluna I recommend getting any of them with a stem because even though the ball stem is cute, its a little more difficult to grab onto. So unless you really like stemless or short stemmed cups, get the traditional stem instead :)
Hopefully this post wasn't too confusing XD