It's important to understand that cups aren't like bras or shoes. There's not just one specific size that fits, nor a standard sizing across the brands. Most can use several cups, or could theoretically use them if they existed in the needed range. Some *absolutely* need to switch between cups depending on their flow and cervix position, but that's not very common. Most of those who have a separate light days cup either originally purchased a cup that was too small/low capacity, or couldn't resist trying out a different brand/colour/stem type that they didn't technically need.
The "goldilocks cup" is generally the best fit from what exists on the market. But for a vast, vast majority of cuppers, any first cup they got was better than disposables.
For many folks, especially with a low cervix and/or heavy flow, finding the right cup is really more about finding the limits of their range, finding the best combination of length and capacity, possibly making tradeoffs sometimes. This is further complicated by the fact that your cervix can sit in the cup and reduce the capacity, or a specific cup may just be a poor fit for no obvious reason, causing leaks and/or discomfort.
The limit is also exploited by the companies, most of which recommend the small(er) sizes to anyone who can wear them, regardless of whether they can also wear the large/wider cups. (Yes, being able to wear different cups also often applies to different sizes of the same brand) Pre-Internet, the expectation was that you'll get the large of the same brand if your flow is too heavy or after you give birth. Nowadays people are very likely to research online and switch brands when getting a second cup.
As it's been said repeatedly, you need to choose by the dimensions, disregarding the official guidelines and especially all the buzzwords like medium/teen size, "XL" or "low cervix version". For each of these, there are other cups that fit the description better. And teens don't necessarily need short/narrow cups, especially if the stem is also short/unusual (ball or ring).
If you have a heavy flow, obviously you need a high capacity cup. But if your flow is light or average, you don't have to choose a low capacity cup, the way you choose pads or tampons with a lower absorbency. (The Mansfield-Voda-Jorgensen scale might be useful)
So, don't stress too much about getting the size "just right". If you have any special needs, you probably know what they are, or you'll know when you look for your cervix. Use the glossary and consider browsing the relevant tags. If you're confused, ask in the community (not in the comments to this post). To answer one of the most common questions, virgins and folks who've had a C-section can typically wear a wide cup if they need it.
Use the size charts to find out which cups are considered short, medium and long. Generally, very few need their cup to be shorter tham 50-52 mm, no matter the exact distance to their cervix. However, a cup may also be too long without actually sticking out (this seems more common with the cups that are 55 mm long or more)
Similarly, cup reviews are of limited usefulness. They're mostly good for the photos, videos and visual comparison, as well as details like stiffness, stem, the number of holes. But quality concerns aside, there's no such thing as a good or bad cup. It all depends on your needs.
The best-known brands are usually the ones that have been on the market longer, or the easiest to get in a particular region, such as the Diva, Keeper, Mooncup (UK/US), Lunette. But actually, since these were the pioneers of cup design, they all fit a limited scope of people. Especially Diva is about as far from one-size-fits-all as you can get.