Some SIZING GUIDELINES
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:
http://menstrual-cups.livejournal.com/tag/faq
Melissa's side-by-side comparison photos:
http://menstrualcups.friendhood.net/f52-moderator-s-photo-library
Melissa's stiffness ratings:
http://menstrualcupinfo.wordpress.com/cup-stiffness-comparison-chart/
Afriska's photos and guidelines:
http://en.afriska.ch/which-one-for-you.html