First we need to recognize that each woman is different 'down there' and also that the vagina can and does undergo shape and size changes based on various factors, which can change with time, age, sexual activity, and are affected also by genetics and hormones. So one size does not fit all and in reality, maybe the two sizes suggested by most menstrual cups are not standard either. With that said, the two suggested menstrual cup sizes should fit most women at nearly any stage of their life.
The conventional wisdom suggests one should buy the small menstrual cup if you haven't given birth yet and the larger cup if you have given birth. This suggestion is based on the idea that the muscles of the vagina are a bit looser or more elastic after giving birth vs. before.
However, conventional wisdom does not work for everyone and it can be a trial and error of judgment for some women.
In addition to the Blossom Cup rim size, there are other matters to consider, which may be more important such as how heavy your flow is and how strong your vaginal walls are.
If you are prone to incontinence due to loose or un-exercised vagina muscles (the kegels), it is best to try first the larger cup. This goes for those who have given birth vaginally, especially more than once, and especially if they suffered/are suffering for issues related to looser vaginal muscles. We would venture to suggest that if your sexual partner is above average or you enjoy larger than average sex toys, to please first consider the larger cup size.
If you are new to the whole insertion idea, and the largest thing that ever made it into your vagina was a tampon, then your best first choice should be the smaller sized cup.
When it is all said and done, please remember this truth, that everyone is different yes but the vagina is an elastic organ and though it can expand to more than 10 cm to accommodate a baby being birthed, it also rapidly shrinks and spends most of its life in this shrunken state where you can insert your middle finger and feel all of your vaginal walls hugging it.
Therefore, after doing your research, go with what makes you feel comfortable because the Blossom Menstrual cup is meant to give us the comfort and confidence to be our best at any time of the month and year.