Well some people are OK with the more narrow cups but I wear ones to fit my breast width the noogleberry xxl cups I use these at first getting suction was awkward and they were the right diameter for me but couldn't get any suction with smaller it was mainly painful crushing and pinching so no smaller ones didn't work for me and that was Xl.
If you already had bigger breasts nbe should be much easier for you you already seen to be doing well.
I don't know what cups your looking at nooglebery dosent have 6in xl is the closest size but then xxl is almost head size so a big size difference if you can find a 6 inch that would be best but dont go wider than your breasts because that will be very difficult to use if you can get suction at all
If you already had bigger breasts nbe should be much easier for you you already seen to be doing well.
I don't know what cups your looking at nooglebery dosent have 6in xl is the closest size but then xxl is almost head size so a big size difference if you can find a 6 inch that would be best but dont go wider than your breasts because that will be very difficult to use if you can get suction at all