Menstrual cup sizes

So I am 18 and i just had a baby vaginally. Once I start getting periods again, I plan to use a menstrual cup. I notice there’s a bigger size for “those over 30 and/or have given birth”. Will I need the bigger size? Or will I be fine with the smaller one? I don’t want to get a big one and it be uncomfortable, but I don’t want to get a small one and leak.