Every body is different. The best way to know which size you should get is by knowing the placement of your cervix (learn how to measure your cervix here), comparing the different options, and by trying it out.

It is measured. The cervix moves and its height or placement changes. The best time to measure your cervix is during your period. It is advisable to measure at the beginning, middle, and end of your period. To measure, find a comfortable position and insert a finger into your vagina and feel for your cervix.
Most use their knuckles as indicators of their cervix height. This may be inaccurate as we all have different finger lengths.
You may use a ruler to accurately measure the length of your finger that was inside of your vagina upon reaching your cervix. 

Yes, if used properly, either a cup or a disc will work for you!
However if you are asking for a point-blank recommendation, R/evolution recommends using a menstrual disc since it does not create a suction.
If you are still unsure, you can consult your doctor, although most women switch to cups and discs right away.

Click here:
MENSTRUAL CUP SIZES
MENSTRUAL DISC SIZES

If you are having a hard time inserting your cup or disc, you might be feeling tense. Relax your body, feel if there is any tension in your vaginal area, and release that tension. While remaining calm, re-insert your cup or disc.
Some also prefer using lubricants.
Just make sure it’s a WATER-BASED lubricant so it will not negatively impact your disc or cups quality and life span.
If it still doesn’t work, remember that great things don’t always come easily! Some users get the hang of it easily while others need more practice and time.
Do not give up right away.
Instead, try again when you feel comfortable enough to do so.
If you need help, don’t hesitate messaging us!
We’re in this together! 

Yes, you may, as long as the lubricant is a water-based lubricant to not catalyze disc or cup deterioration.

You can have a worry-free period for up to twelve (12) hours! When you feel like it’s time to dump the contents of your cup or disc, just remember 3Rs:
REMOVE, RINSE, and REINSERT!
Don’t forget to wash your hands before and after!

Absolutely! Your cup or disc will remain unbothered by swimming!
Plus, you don’t have to worry about your menstruation leaking or dripping because your cup/disc has got you covered!
Swim all you want, R/ebel!