I've put all of my answers in one place:
(a work in progress)
Scroll down if you want to jump to the charts.
Don't pick a brand or a size first. Go by actual dimensions.
EVERY brand says that their Small is for younger users who have not had children that and their Large is for those over a certain age and/or have given birth. But what they are called doesn't fall within any range of physical sizes. And of course, every company is going to tell you that their cup will fit you better than their competitors'.
LENGTH is probably your most important measurement. If you pick a cup that is too long, it will probably shift, leak, feel like it's poking out and be uncomfortable. If you pick a cup that is too short, your cervix might fill it, leaving very little room for flow and/or you might have a hard time reaching it for removal.
Next period, reach in to feel how long your vagina is.
If you can reach the end (not necessarily your cervix) easily, it's on the short side.
If you have to reach really deep or still can't reach it, it's long.
If it's in between, aim for a medium length cup.
If you're not sure about length, logic says that picking one that's "medium" will have a better chance of fitting than one from either end of the length spectrum.
WIDTH is measured at the rim.
If you are young and/or new to inserting things into your vagina, you will probably want a narrow to medium width cup.
If you are comfortable with insertion and/or older and/or have had children, a medium to wide cup will probably suit you.
Once you have an idea of length and width preferences, use the size charts to eliminate the ones you don't think will fit. And then select the one that calls to you from the ones that remain.
It's not an exact science but getting into the right ball park will increase your chances of success greatly.
The SHAPE of the cup probably has a greater influence than rim width on how "big" or "small" a cup feels so also compare photos of those that fall into your fitment parameters. A cup's STIFFNESS also plays a role.
Nearly all cups are made of medical grade silicone. Exceptions are the Keeper, which is made of latex rubber and the MeLunas, which are made of TPE (thermoplastic elastomers.)
Being "tight" is a function of your pelvic floor (PC) muscles. Cups sit above them. You need to relax them for insertion and removal and then you can go back to holding them as tightly as you normally do. Above them, your vaginal wall muscles become less elastic as you age and/or have children.
There are more charts by other list members available at:
Melissa's side-by-side comparison photos:
Melissa's stiffness ratings:
Afriska's photos and guidelines:
TECHNICAL NOTE: This chart MIGHT get truncated (cut off) when viewed on a handheld device like a phone or a tablet. 48mm is the widest width and then there is a column of words that describe length. If you can't see them, then look at it again on a "real" computer to see the whole thing.
If it still gets truncated, try this: Hold the Ctrl button and hit the - (dash or minus sign) to widen your view. To get back to your original view, Hold both the Ctrl button and the shift button to hit the + (plus sign.) "Ctrl -" to zoom out, "Ctrl +" to zoom in.
There are four width and several length categories as denoted by the gray and white shading.
Note that "Medium" length and width are not in the middle.
|35 Super Short||S Shorty MeLuna||35 Super Short|
|38||M Shorty MeLuna||
|40 Really Short||S Mini MeLuna||40 Really Short|
|41||L Shorty MeLuna||41|
|44||XL Shorty MeLuna||44|
|45||S MeLuna||S Iris
|S Lybera=42.5||M Mini MeLuna||L Lybera||45|
|46 Very Short||S JuJu
|S Lena||46 Very Short|
|47||S CupLee||S FleurCup
|48||S Angelcup||M MeLuna
|49 Short||S Yuuki||49 Short|
|52 Medium||S Femcup||L Lena
|53||S Miacup||L CupLee||L Bella=45.5
|55 Long||S Jasmine||M Naturcup||S Gaia||L Rainbow
|L Femcup||L Naturcup
|56||S MissCup||L MissCup||Natural Mamma||L Gaia||XL MeLuna||56|
|57||S Diva||L Mami||L Diva||L Luv Ur Body||57|
|58 Very Long||58 Very Long|
|62 Really Long||M Luv Ur Body||62 Really Long|
|(even longer)||S Lily = 40x77||L Lily = 42.5x82||(even longer)|
|PRIMARY sort||SECONDARY sort||FINAL sort|
says to brim
says to holes
|Tampons||Light||up to 6|
|Tampons||Regular||6 to 9|
|Keeper & Moon Cup||Small||41x54||-||-||10, 13, 15|
|Tampons||Super||9 to 12|
|UK Mooncup/MCUK||Small||43x50||-||-||12.5, 13, 15|
|UK Mooncup/MCUK||Large||46x50||-||-||14, 15|
|Tampons||Super Plus||12 to 15|
|Keeper & Moon Cup||Large||44x54||-||-||15|
|Tampons||Ultra||15 to 18|
|Lady/Color Cup||Small||40x46||21.2||-||11, 15, 15.5|
|Lily Cup||Large||42.5x88||25||no holes||-|
|Yuuki||Small||42x49||25||"13 to safety ring"||19, 22.5|
|Lunette||Small||41x47||25||-||20, 21, 22.5|
|Lady/Color Cup||Large||46x53||25||-||20, 22.5, 26|
|MPower||One Size||47x54||-||-||27, 30+|
|SheCup||One Size||44x54||28||16||16, 17.5|
|Femmecup||One Size||45x50||30||-||15, 18|
|Luv Ur Body||Medium||45x62||31|
|MeLuna (old)||Large||45x54||32||24||22.5, 24|
|Natural Mamma||One Size||44x56||33||27||-|
|Luv Ur Body||Large||48x57||34.5|
|Yuuki||Large||47x55||37||"25 to safety ring"||29, 35|
|LunaCup||Medium (No small?)||???||??||-||-|